ATMOSPHERE is the name applied to the invisible elastic medium which surrounds the globe of the earth to an unknown height. The fluid of which it is composed is usually distinguished by the name of air.

It is well known that the pressure of the atmosphere is the cause of the rise of the mercury in the barometer. Now the mean height of the barometer at the level of the sea is 29∙82 inches. Hence it follows that the whole weight of the atmosphere is equal to that of a sphere of mercury covering the whole surface of the globe, and extending to the height of 29∙82 inches. The specific gravity of mercury is 13∙568, and a cubic inch of water at the temperature of 60° weighs very nearly 252∙5 grains. Hence a cubic inch of mercury will weigh 3425∙92 grains, or 0∙48956 lb. avoirdupois; and a column of the atmosphere reaching from the surface of the earth to the utmost height to which that elastic fluid reaches, and whose base is a square inch, weighs about 14*6 lbs. avoirdupois, or exerts a pressure equal to 14*6 lbs. upon the earth or any substance on which it rests.

The atmosphere consists chiefly of air, an elastic fluid composed of a mixture of four volumes of azotic with one volume of oxygen gas. Now, from the most exact experiments hitherto made, it follows, that at the temperature of 60°, and under the mean pressure of the atmosphere, a cubic inch of air weighs 0∙311446 grains. If, therefore, the air were everywhere of the same density, the height of the atmosphere above the surface of the earth would be 328,021 inches, or 27,335 feet, or 5∙17 miles.

But air is an elastic fluid, the particles of which repel; each other with a force varying inversely as the distance 1 of the centres of the particles from each other. It is obvious, from this property of air, that its volume, and consequently its density, will depend upon the pressure. The greater the pressure, the smaller the volume. Those portions of the atmosphere that are in contact with the earth are pressed upon by the whole portion above them. The air at the top of a mountain is pressed upon by all the air above it; while all that portion below it, or lying between the top of the mountain and the surface of the sea, exerts no action on it whatever. This will be better understood by the following diagram. Let ABCD be conceived to be a section of a vessel extending from the surface of the earth at AB to the limit of the atmosphere CD, and filled with air. Let this pillar of air be divided into an infinite number of equal sections, each exceedingly thin, by the planes er, gh, ih, lm, no, &c. Since the density of air is always as the compressing force, it follows that the density of the stratum ABe∕ ’ is to that of the density of the stratum efgh as ç/CD to g∙ACD. So that the difference between the pressures on er and gh is equal to the quantity of air efgh. For the same reason, the difference be tween the pressures on gh and ih is equal to the quantity of air ghih ; and this ratio holds to the very limit of the atmosphere. Therefore the densities of air in these spaces are proportional to the quantities of which they themselves are the differences. Now, when there is a series of quantities whose terms are proportional to their own differences, both those quantities and their differences are in geometrical progression; therefore the densities of the strata of air ABc∕i efgh> ghih, ihlm, &c. are in geometrical progression.

It is equally obvious that the heights of these equal spaces above AB, the surface of the earth, are in arithmetical[7:4:133] progression. If er be 1 inch above the earth’s surface, gh will be 2 inches, ik will be 3 inches, and so on. From all this we derive this remarkable conclusion, that if the altitudes above the surface of the earth be taken in arithmetical progression, the densities of the air at these altitudes will be in geometrical progression decreasing. If at a certain altitude above the earth’s surface the density of the air be one half what it is at the surface of the earth, then at twice that altitude the density will be reduced to one fourth of the density at the earth’s surface, and so on.

We can calculate the density of the atmosphere at all heights above the earth’s surface very readily by means of a table of logarithms; for logarithms constitute a set of numbers in arithmetical progression annexed to another set of numbers which are in geometrical progression. If, therefore, we take logarithms to represent the heights, the numbers to which these logarithms are attached will represent the corresponding densities of the air. Suppose the density of the air at one mile above the surface of the earth to be represented by unity; then, from the common tables of logarithms, we easily deduce the following densities at greater heights:

Height. Density.

1 mile 1*

2 0∙7943

3 0∙6309

4 0∙5011

5 0∙3981

6 0∙3163

7 0∙2511

8 0∙1995

9 0∙1585

10 0∙1260

So that at 10 miles the density of the air is only ∣-th of its density at one mile above the surface of the earth.

These observations are sufficient to show that the height of the atmosphere above the surface of the earth must greatly exceed five miles; but how much, we have no data to enable us to determine.

Air possesses the property of refracting light; that is, of bending it from a right line, and making it move in a curve. The consequence of this property is, that the sun continues visible for some little time after he sets, and is seen also a short time before he rises. Nor are we deprived of all the benefits of his rays so long as any of them are capable of reaching the utmost limit of the atmosphere. This light, brought to the surface of the earth by the refracting power of the atmosphere, is called twilight; and mathematicians have endeavoured to determine the height of the atmosphere by observing how many degrees below the horizon the sun must sink before twilight ends. The result of this calculation is, that, at the height of 45 miles, the atmosphere has no sensible power of refracting light. Its rarity therefore at that height must be very great.

There is another way in which an estimate may be formed of the height of the atmosphere. That height must ultimately depend upon the degree of rarity which air is capable of bearing. The particles of air repel each other with forces varying inversely as the distances of the centres of these particles from each other. Now there is no great difficulty in rarefying air by means of a good air-pump, till it is capable of supporting a column of mercury only γθth of an inch in height. In such a case air is rarefied about 300 times, and the distance between the centres of its particles is increased seven times; consequently the force of repulsion between these particles is reduced to ^th of what it is when air is of its mean density. If we suppose that this is the utmost limit to the rarefaction of air (a supposition not at all likely to be strictly true), we are entitled to infer that the atmosphere extends to the height of 40 miles, with properties yet unimpaired by extreme rarefaction.

If matter be infinitely divisible, the extent of the atmosphere must be equally infinite. But if air consists of ultimate atoms no longer divisible, then must the expansion of the medium composed of them cease at that distance where the force of gravity downwards upon a single particle is equal to the resisting force arising from the repulsive force of the particles. If the air be composed of indivisible atoms, our atmosphere may be conceived to be a medium of finite extent, and may be peculiar to our planet; but if we adopt the hypothesis of the infinite divisibility of matter, we must suppose the same medium to pervade all space, where it would not be in equilibrio, unless the sun, the moon, and all the planets, possess their respective shares of it condensed around them, in degrees depending on the force of their respective attractions.

It is obvious that the atmosphere of the moon (supposing it to have any) could not be perceived by us; for, since the density of an atmosphere of infinite divisibility at her surface would depend upon the force of her gravitation at that point, it would not be greater than that of our atmosphere when the earth’s attraction is equal to that of the moon at her surface. Now this takes place at about 5000 miles from the earth’s surface,—a height at which our atmosphere, supposing it to extend so far, would be quite insensible.

But since Jupiter is fully 309 times greater than the earth, the distance at which his action is equal to gravity must be as V ’ 309, or about 17∙6 times the earth’s radius; and since his diameter is nearly 11 times greater than

17∙6 . . . .

that of the earth, 11 = 1∙6 times his own radius will be the distance from his centre at which an atmosphere equal to our own should occasion a refraction exceeding 1°. To the fourth satellite this distance would subtend an angle of about 3° 37'; so that an increase of density to 3½ times our common atmosphere would be more than sufficient to render the fourth satellite visible to us when behind the centre of the planet, and consequently to make it appear on both sides at the same time. It is needless to say that this does not happen, and that the approach of the satellites, instead of being retarded by refraction, is regular till they appear in actual contact; showing that there is not that extent of atmosphere which Jupiter should attract to himself from an infinitely divisible medium filling space.

If the mass of the sun be considered as 330,000 times that of the earth, the distance at which his force is equal to gravitation will be √330,000, or about 575 times the earth’s radius; and if his radius be 111*5 times that of 575

the earth, then this distance will be —— = 5∙15 times lll∙o the sun’s radius. But Dr Wollaston has shown, by the phenomena attending the passage of Venus very near the sun on the 23d May 1821, that the sun has no sensible atmosphere. For the apparent and calculated place of that planet were the same when the planet was only 53' 15" from the sun’s centre. Μ. Vidal of Montpellier observed Venus on the 30th May 1805, when her distance from the centre of the sun was only 46' of space; and the apparent and calculated positions of that planet corresponded. These observations leave no doubt that the sun has no sensible atmosphere, and of course are inconsistent with the notion of the infinite divisibility of the matter of our atmosphere.
[7:4:134]

But if air consist of atoms incapable of further division, it is obvious that the height of the atmosphere has a limit, and that limit is the place where the gravitation of the atoms of air just balance the force of their repulsion. The exact situation of this limit we cannot assign; but it cannot far exceed the height of 45 miles above the earth’s • surface. Nor are the objections to this determination drawn from meteors of sufficient weight to overturn the force of the arguments just adduced. Dr Halley observed a meteor in the month of May 1719, whose altitude he computed to be between 69 and 73∣ English miles, its diameter 2800 yards, and its velocity about 350 miles in a minute. The celebrated meteor which appeared on the 18th August 1783 was not less than 90 miles above the surface of the earth. Its diameter must have been at least equal to that of Dr Halley’s meteor, and its velocity certainly not less than 1000 miles in a minute. We know too little about the nature of these meteors to connect them with the earth’s atmosphere. Indeed their velocity would lead to the conclusion that they were revolving round the earth like satellites. The light which they emitted, or the state of combustion which they exhibited, we cannot explain. But the same difficulty occurs to account for the volcanoes which have been seen in the moon, though we are quite certain that her atmosphere is far too rare to support combustion.

The ancients thought that air constituted one of the four elements, from which, in their opinion, all things originated; and this doctrine continued prevalent till after the year 1774. It was during that year that Dr Priestley first discovered oxygen gas, and showed it to be a constituent of air. He determined several of its most remarkable properties, and called it dephlogisticated air, from a notion he entertained that it was air deprived of phlogiston. When azotic gas, the other constituent of air, was discovered soon after, the difference between its properties and those of oxygen gas could not fail to strike the most careless observer. Bodies burn more rapidly, with much greater splendour, and with the evolution of much greater heat, in oxygen gas than they do in common air; while in azotic gas they cannot be made to burn at all. Animals breathe oxygen gas without inconvenience, and they live much longer when confined in a given bulk of it, than when in the same volume of common air; but in azotic gas animals cannot live at all. When plunged into it they die of suffocation, precisely as they would do if plunged under water. Dr Priestley considered oxygen gas as the pure elementary principle of the ancients: common air was oxygen united to a certain quantity of phlogiston, while azotic gas was oxygen saturated with phlogiston.

Scheele discovered both oxygen and azotic gas, without any knowledge of what had been done in Britain; and he first drew the proper consequences from his experiments. Air, in his opinion, consists essentially of a mixture of two distinct elastic fluids, namely, oxygen and azote. He determined the properties of each, and made a set of experiments to ascertain the relative volumes of each contained in the atmosphere. The result of these experiments led him to the conclusion that air is a mixture of
[table]

Lavoisier drew the same conclusions as Scheele had done; and he assures us that he did so before he knew any thing of the researches and discoveries of the Swedish chemist; and, what is very remarkable, he deduced the same volumes of each gas, as constituents of the atmosphere, as Scheele had done. His experiments were made in the same way as Scheele’s, and no doubt the mistakes of both had the same origin.

It was in 1782 that Mr Cavendish, by a careful analysis of the air in the neighbourhood of London, repeated frequently, and continued for a whole year, determined that the volume of oxygen gas in atmospherical air is a good deal smaller than Scheele and Lavoisier had made it. He found the constituents of air to be—
[table]

He found that these proportions never varied, though he analyzed air at different periods of the day, and during all the different seasons of the year. Hence it followed that the conclusions drawn by Dr Ingenhousz and others, that air differs in the proportion of oxygen gas which it contains in different parts of the earth, and that the salubrity of different places is connected with this difference, were erroneous.

These experiments of Mr Cavendish were published in the Philosophical Transactions for 1783; but they continued for many years unattended to. The determination of Scheele and Lavoisier was universally adopted; and the notion of Dr Ingenhousz, that the proportions between the oxygen and the azote vary in different places, was also adopted. At last, in 1802, Berthollet announced that he had frequently analyzed the air in Egypt, by absorbing its oxygen by means of a stick of phosphorus, and that he had always found it a compound of
[table]

Davy about the same time announced that he had tried air from the coast of Africa, from Cornwall, and from the neighbourhood of Bristol, and had uniformly found it composed of 79 volumes of azotic gas and 21 volumes of oxygen gas. It was soon after analyzed in Edinburgh, in North America, and in France, with the very same results. Gay-Lussac and Humboldt made a set of careful experiments to determine the exact proportions of the two constituents, and confirmed the ratio of 21 volumes of oxygen gas and 79 volumes of azotic gas. This ratio has been generally adopted by chemists.

But there is a circumstance which cannot avoid striking an attentive observer; namely, the very near approach of these numbers to 80 volumes azotic and 20 volumes oxygen gas, or four volumes azotic and one volume oxygen gas. It has been deduced from a great variety of unexceptionable experiments, that a volume of azotic gas is equivalent to an atom, while half a volume of oxygen gas is equivalent to an atom. Hence four volumes azotic and one volume oxygen gases are equivalent tô four atoms azote and two atoms oxygen, or to two atoms azote and one atom oxygen. If, therefore, we were to admit that air consists of 80 volumes azotic and 20 volumes of oxygen gases, it would follow that it is a compound of two atoms azote and one atom oxygen. There is no evidence, indeed, that in air the oxygen and azotic gases are chemically combined; the phenomena of air rather lead to the conclusion that the two elastic fluids are merely mixed. And the hypothesis of Dalton, that the particles of elastic fluids only repel particles of their own kind, but that two elastic fluids are not mutually elastic to each other, will enable us to account for their remaining intimately mixed, though not chemically united.

It may seem immaterial, then, whether air be a mixture of azotic and oxygen gases in the proportion of two atoms of the former to one atom of the latter, or in the pro [7:4:135]portion ( as c∏emists at present think) of 1∙975 atom azote and 1∙05 atom oxygen; or, as fractions of atoms can hardly be admitted, of 79 atoms of azote and 42 atoms of oxygen. But certainly the constitution of air must appear much simpler, and therefore much more agreeable to contemplate, when viewed as consisting of the simple ratio of two to one, than in the much more complex ratio of 79 to 42. The writer of this article was induced in consequence to make a set of experiments with all possible attention to accuracy, in order to deduce the volume of oxygen gas which it contains. The air was collected in a grass field at some distance from houses and trees; and this spot was selected as likely to furnish air containing as great a proportion of oxygen as it ever contains, because grass and vegetables in general are supposed to make up the waste of oxygen which the air sustains by the processes of combustion and respiration,^[1. It is easy to show, that whether the oxygen thus withdrawn from the air be supplied or not, no alteration in the proportions of the constituents amounting to one millionth part can possibly have taken place. It would therefore be quite insensible as far as our means of estimating it can go. ] and because the quantity of carbonic acid which it was likely to contain would be too small to make any sensible alteration in the experiments.

Ten measured volumes of this air were placed successively in a small glass jar, over mercury, together with a stick of phosphorus. After the oxygen gas had been removed by the phosphorus, the air was washed, and then measured. The following table exhibits the result of these ten trials. 100 volumes of air consist of

Volumes of Volumes of

Azotic Gas. Oxygen Gas.

1 80∙927 19∙073

2 79∙246 20∙754

3 80∙504 19∙496

4 79∙532 20∙468

5 79∙85I 20∙149

6 79∙652 20∙348

7 79∙374 20∙626

8 80∙770 19∙230

9 79∙843 20∙157

10 80∙028 19∙972

Mean, 79∙9735, ............. ...20∙0265

The mean of these ten trials gives us air, a mixture of 79∙9735 volumes of azotic and 20∙0265 volumes of oxygen gas. Now, this approaches as near 80 volumes of azotic and 20 volumes of oxygen gas as the mode of experimenting permitted, for the measure employed to determine the volume of the gases was about half an inch in diameter. .

It was found, also, that when 100 volumes of air were mixed with 42 volumes of hydrogen gas, and an electric spark passed through the mixture, the diminution of bulk by the explosion was precisely 60 volumes; and this in three successive experiments. Now, this diminution is owing to the oxygen of the air uniting with the hydrogen gas and forming water, and water is a compound of two volumes hydrogen and one volume oxygen gas. The third part of the diminution, therefore, gives us the quantity of oxygen gas consumed. Now, the third of 60 is 20, which constitutes therefore the volumes of oxygen gas in 100 volumes of air.

If we employ less than 42 volumes of hydrogen gas, the whole oxygen of the air is not consumed. Thus, when we employ a mixture of 100 volumes of air and 40 volumes of hydrogen gas, the diminution of volume after combustion is only 57 volumes, which would indicate 19 volumes of oxygen instead of 20 in the air; so that one volume of oxygen in this case has escaped combustion. When we mix more hydrogen than 42 volumes with 100 volumes of common air, the diminution of volume is somewhat greater than 60, and it goes on increasing slowly till the volume of hydrogen gas is equal to that of the air. 100 volumes of air being mixed with 100 volumes of hydrogen gas, and fired, the diminution amounted to 64 volumes. This would raise the amount of the oxygen in 100 volumes of air to 21∣^ volumes.

It is obvious from these experiments that absolute certainty cannot be obtained by firing mixtures of air and hydrogen gas. The reason, doubtless, is, that when the surplus of hydrogen is considerable, a little of it unites with the azotic gas of the air, and forms ammonia, which, being absorbed by the water over which the explosion is made, occasions a greater diminution of bulk than would have proceeded from the simple union of all the oxygen of the air with hydrogen gas.

Knowing the composition of air to be 80 volumes azotic and 20 volumes oxygen gas, and knowing the atemic weight of these bodies to be azote 1∙75 and oxygen 1, we can easily deduce the specific gravity of azotic and’ oxygen gases, reckoning that of air unity, in the following manner :—

100 parts of air by weight must be a compound of oxygen 22∙22= a, azote 77∙77=δ.

Let zr=specific gravity of oxygen gas, 2∕=specific gravity of azotic gas, a? + 4?y _ n „ .

—— 1 and x = 5 — 4?/ 5 j

. 7 . 4α?/

X : 4?/:: a : b and χ = —A-, j b

Hence 5 — 4∕∕=^v^, from which we deduce j b 5b 5×77∙77 .

y∑∑l “ ∩∙Q729

j 4«+46 4 × 22∙22 + 4 × 77∙77 ~~

and χ =5 — 4?/=5 — 3∙8888= 1∙I 11İ.

Thus, the specific gravity of these two gases is as follows :—

oxygen gas 1∙1111, azotic gas 0∙9722.

That the specific gravity of these two gases hitherto determined by the most careful experimenters is inaccurate, is obvious from this, that when we calculate the specific gravity of air from them, it never comes out unity, but usually higher than unity.

Besides oxygen and azotic gases, air contains likewise a little carbonic acid gas. Who first made that remark we do not know; but n could not avoid being inferred as soon as the cause of the difference between caustic and mild alkali came to be known. Chemists at first stated the volume of carbonic acid gas in the atmosphere at 1 per cent., but this determination was not founded on any accurate experiments. Mr Dalton found the quantity much smaller than had been stated by preceding experimenters. He observed, that if a glass vessel, filled with 102,400 grains of rain water, be emptied in the open air, and 125 grains of lime water poured in, and the mouth then closed, by sufficient time and agitation the whole of the lime water is just saturated with the carbonic acid which it finds in the inclosed volume of air; but 125 measures[7:4:136] of lime water require 70 measures of carbonic acid gas to saturate them. Hence it follows that 10,000 volumes of air contain 6 ’ 8 volumes of carbonic acid.^[2. Phd. Mag. (1st senes), xxiii, 354. ] Thenard ascertained by means of barytes water that 10,000 volumes of air which he examined contained 3∙91 volumes of carbonic acid.^[3. Traite Eiern, de Chirn. i. 303. ] This is little more than half the quantity found by Mr Dalton. But by far the most complete set of experiments on the volume of carbonic acid gas in the atmosphere was made by M. Saussure. He abstracted the carbonic acid from given volumes of air by means of barytes water. The carbonic acid was estimated by dissolving the precipitated carbonate of barytes in muriatic acid, and throwing down the barytes in the state of sulphate from the solution. The sulphate of barytes, ignited and weighed, easily furnished the weight of carbonic acid; and this weight, together with the known specific gravity of carbonic acid gas, furnished the data for determining its volume. The experiments were continued for two years. Sometimes (indeed most commonly) the air examined was collected at Chambeisy, a meadow about three fourths of a league from Geneva, elevated about 52 1 - feet above the lake, and distant from it 820 feet. Its elevation above the level of the sea is 1272 feet. It is dry, open, and consists of a clay soil, a little inclined. The mean quantity of carbonic acid gas found in 10,000 volumes of air deduced from 104 observations made during both day and night, and at all seasons, was 4∙15 volumes. The greatest quantity was 5∙74 volumes, and the smallest 3∙13 volumes. The quantity of this gas is affected by rainy weather; for when the soil is soaked with water, it has the property of imbibing this gas. Hence in rainy seasons the quantity of carbonic acid gas in the atmosphere is usually rather less than in dry seasons. On the contrary, it would seem, from the observations of Saussure, that a continued frost has a tendency to augment the quantity of carbonic acid in the atmosphere, doubtless because the frozen and dry soil does not possess the property of absorbing it.

Air taken from the surface of the lake of Geneva, at an elevation of four feet, was found in general to contain less carbonic acid than the air of Chambeisy; but the same kind of variation, depending on the season of the year, was observable in both. Air from the streets of Geneva, on the contrary, was found rather more loaded with carbonic acyl than the air above Chambeisy; but the difference was not great. The volume of carbonic acid in air from the tops of mountains was found to be rather greater than that of carbonic acid in air from the low country; and this difference is more remarkable in rainy than in dry weather.

Wind has rather a tendency to augment the quantity of carbonic acid in the air in low situations, doubtless by mixing together the air on the mountains and in the valleys; but the difference is so small that it only becomes perceptible by a long series of observations.

In general, the air over plains contains a greater proportion of carbonic acid during the night than during the day; but this difference is not nearly so great in winter as in summer. Indeed during that season of the year the quantity is often as great or even greater during the day than during the night.^[4. Ann. de Chirn. et de Phyt. xliv. 5. ]

The reason why the quantity of carbonic acid gas in the atmosphere is greater in the superior strata than at the surface of the earth, is, probably, that vegetables have the property of absorbing it and applying it to the purposes of vegetation. Hence, doubtless, the reason why it does not increase, notwithstanding the prodigious quantity of it constantly thrown into the atmosphere by the breathing of animals and the combustion of fuel.

If we admit the mean volume of carbonic acid gas in air to be 0*000415, then the true component parts of lθθ; volumes of air will be

Azotic gas 79∙9668

Oxygen gas 19 ’ 9917

Carbonic acid gas 0 ’ 0415

100.

This, doubtless, will somewhat modify the specific gravity of oxygen and azotic gases; but the alteration is too insignificant to claim any attention.

Besides these three constituents, there is a fourth, name-, ly, the vapour of water, from which air is never altogether ∣ free, and the proportion of which, owing to causes which! cannot at present be fully explained, is continually altering. The consequence of this is, that if we weigh air in its natural state ever so often, we shall hardly ever find its weight in any two consecutive experiments altogether the same. It is sufficiently known that water evaporates spontaneously at all temperatures, and mixes with air in the state of an invisible elastic fluid known by the name of vapour or of steam when its elasticity is so great as to balance that of the atmosphere. The elasticity of vapour varies with the temperature. At 32° it is capable of supporting a column of mercury 0∙2 inch high. At 80° it supports a column of 1 inch, at 163° of about 10 inches, at 180° of about 15 inches, and at 212° of about 30 inches. Now, the quantity of it which can exist in the atmosphere at the same time is proportional to this elasticity.

Mr Dalton has shown, that if p = pressure of the atmosphere in inches of mercury, y = elasticity of vapour contained in the atmosphere, x = volume of dry air in 100 volumes of the given atmospherical air; then 100

— 100; consequently χ — p . p ~ p p -f

Let air be saturated with moisture at 32°. In that case we have p ¾ = ⅛⅛ = 1 * 00δ71 2

* = Fδ≡ = "∙ 333ι so that the volume of vapour (supposing its specific gravity 0∙625) in 100 volumes of such air will amount to 0∙666, which is just τ ^-θth part of the volume of the air.

At the temperature of 60° f = 0∙52, we have therefore px 30 x

? = ōri 4 77 = lθθ, and x = 98*267; so that the vo- p—f 29∙48

lume of vapour capable of existing in the atmosphere at 17∙33

θ0° is 1θθ,θθθ of* the atmosphere.

The highest temperature that the writer of this article has had an opportunity of witnessing in Great Britain was 93°. At that temperature f = 1∙5

px 30ae

~~ f = -5√l’ = 10θ> and x = 95; p—f 28∙5

so that the volume of vapour capable of existing in the

5 1

atmosphere at such a temperature is ¶— or —.

JL W 4u

To determine the volume of vapour in the atmosphere [7:4:137]at any particular time, various instruments have been contrived, called hygrometers, some more and some less exact; but the simplest and most accurate method of all is that employed by Leroi, and afterwards by Mr Dalton. Take a glass tumbler, as thin as possible, and fill it with water somewhat colder than the temperature of the air at the time. Observe if vapour be condensed on the outside of the tumbler; if not, the water employed is not cold enough for our purpose. Its temperature must be lowered either by putting pieces of ice into it or by dissolving in it some carbonate of soda or sulphate of soda, retaining their water of crystallization, but in powder. If vapour condense on the outside of the tumbler, pour out the water into another glass and wipe the outside of the tumbler dry. When the temperature of the water has had time to be a little elevated, pour it into the tumbler again, and observe whether moisture condense on the outside of the glass. If it do, pour out the water, wipe the outside of the tumbler again, and repeat the process when the water has become a little warmer. This method of proceeding is to be persevered in till you find the temperature at which the moisture just ceases to be condensed on the glass. That temperature, by means of the following table, will enable you to determine the volume of vapour (of the specific gravity 0'625) existing in the atmosphere at the time that your observation is made.
[table]

Suppose we find that the temperature of the water when moisture ceases to be condensed on the outside of the tumbler is 40°, we look into the above table, and opposite to 40° in it, we find 0∙26 inch of mercury. This denotes the force of the vapour contained in the atmosphere at the time our experiment was made. Suppose the barometer to be standing at 30 inches, then the volume of vapour in the atmosphere is 3⅜⅞7y, or nearly τ ∣ 3 - of the volume of the atmosphere.

The absolute quantity of vapour in the atmosphere is usually greatest in summer on account of the temperature being so much higher; but the moisture or dryness of the air does not depend so much upon the absolute quantity of vapour which it contains, as on its approach to saturation. Suppose the temperature of the air to be 60°, and that a tumbler filled with water of 60° condenses water on its outside. This would indicate a force of vapour equal to 0∙52 inch of mercury. Now, as this is the force of vapour at 60°, it is clear that as much vapour exists in the air as is possible at that temperature. The air is saturated with moisture, evaporation cannot go on in it, and moisture will be deposited upon all bodies the least colder than the air itself. Such a state of things takes place pretty frequently in this country during winter, though rarely during summer. Hence the atmosphere is moister during winter than during summer, though the absolute quantity of vapour which it contains may be much le is.

With respect to the quantity of vapour in the atmosphere we have still very few data. Mr Daniel, in his book on Meteorology, has given a table of the force of vapour in the atmosphere at London for three years. The following table gives the mean force of vapour in Glasgow during every fortnight, from May 1823 to February 1824:

Force of Vapour in Inches of Mercury.

1823. May, 1st fortnight 0∙2707

2d 0∙3494

June, 1st 0∙3052

2d 0∙2822

July, 1st 0∙3526

2d 0∙3819

August, 1st 0∙4000

2d 0∙3546

September, 1st 0.3790

2d 0∙3404

October, 1st 0∙3233

2d 0∙3026

November, 1st 0∙2808

2d 0∙3230

December, 1st 0∙2449

2d 0∙2453

1824. January, 1st 0∙2481

2d 0∙2578

February, 1st 0∙-2468

2d 0*2170

The mean force of vapour in the atmosphere in Glasgow is nearly 0∙3 inch, which indicates the quantity of vapour capable of existing in the atmosphere at 45°. Now the mean temperature of Glasgow is 47° ∙75. It is obvious from this that the atmosphere in that part of Scotland is moist.

It would be very interesting if we knew the force of [7:4:138]vapour at all the different seasons in the torrid zone. The regularity of the weather in these climates, and the little alteration in height of the barometer, would make a set of such observations particularly valuable. They would probably throw more light on the theory of rain than has yet been done. The only set of observations which we have seen on the quantity of vapour in countries approaching the torrid zone, are the following by Dr Heinecker, in which he gives the maximum and minimum dew points at Funchal in Madeira (lat. 32⅛ N.), during the year 1828. which in that part of the world was remarkably dry.

Maximum. Minimum. Rain in Inches.

January 65° 50° 4 ’ 08

February 56 50 1 ’ 64

March 65 48 1'68

April 63 45 3∙35

May 69 51 2∙14

June 70 54 0 ’ 21

July 72 61 0∙10

August 73 63 0

September...75 69 1 ’ 39

October 74 56 0

November....72 ’ 5 54 2 ’ 56

December....67 50 0∙52

17∙67^[5. Brewster’s Journal (new series), i. 34. ]

These observations are not sufficient to give us an accurate notion of the state of the atmosphere at Funchal. The highest dew point is 75°, indicating a force of vapour amounting to 0 ’ 8581 inch, so that the vapour at that time in the atmosphere was nearly 3⅛-th of its volume. The lowest dew point is 45°, indicating a force of vapour amounting to 0∙3059 inch, so that the vapour at that time in the atmosphere was nearly τ -Lθ of its volume. The mean dew point deduced from the maxima and minima is 61°, indicating a force of vapour of 0∙5377 inch, or a quantity of vapour amounting to nearly ⅛th of the volume of the atmosphere. The mean temperature of Madeira is 66° ∙3. Now since the mean dew point is 61°, it is obvious that the atmosphere over Madeira is much drier than at Glasgow, though the absolute quantity of vapour which it contains is much greater.

Thus the atmosphere is a mixture of at least four different elastic fluids, namely, azotic gas, oxygen gas, carbonic acid ĝas, and the vapour of water. Doubtless all other gaseous bodies and many vapours exist in it also, but in too small quantities to be discovered by the most delicate tests that we have it in our power to apply. These different elastic fluids are mixed equably together; and though there be a considerable difference in their specific gravities, that difference has no tendency to cause them to separate. The reason of this equable mixture was first pointed out by Dalton. It depends upon a principle not yet generally recognised, but of the existence of which recent observations leave little doubt. This principle is, that the particles of elastic fluids are not mutually elastic to each other. The particles of oxygen repel the particles of oxygen, the particles of azotic gas repel those of azotic gas; but a particle of oxygen does not repel a particle of azotic gas. Hence, wheΛ a gas issues from an orifice into a space filled by another gas, it rushes precisely as if it were flowing into a vacuum. But the full development of this important principle belongs to the article Pneumatics.

A great deal has been written about the salubrity of the atmosphere in different countries. It has been supposed, and is still believed, that the average length of life in different places depends chiefly upon the state of the atmosphere. It has been generally admitted that the atmosphere is frequently a vehicle by which diseases are communicated; and the prevalence of certain epidemic diseases, as the plague and the yellow fever, in particular places, at particular seasons, has been accounted for in this way. But no satisfactory evidence has ever been adduced to satisfy us of the accuracy of these opinions. The constituents of the atmosphere, azotic and oxygen gases, never undergo any sensible change in their proportions. Carbonic acid varies somewhat, but its proportion is always so small that it cannot be considered as a source of disease. The proportion of aqueous vapour is much more variable, and there can be no doubt that it may have an effect upon individuals predisposed to certain diseases. Consumptive patients suffer much more when they breathe a very moist than when they breathe a dry atmosphere. There are few places in the habitable globe, if there be any, where the atmosphere is constantly saturated with moisture; and it cannot prove very injurious, even to consumptive patients, except when in this state. Besides, that a very dry atmosphere is not the best adapted for the continuance of health, is obvious from the sufferings to which those who inhabit the west coast of Africa are liable during the prevalence of the sirocco, a wind So excessively dry that even the wood of the floors shrinks in consequence of its action.

Nothing has been more completely ascertained than that marshy countries are subject to intermittent fevers, and that the malignancy of these intermittent fevers increases with the heat of the climate. These diseases disappear when the marshes are drained, and therefore are connected with moisture; but that they are not owing to mere moisture, is obvious from this, that they do not appear at the sea-shore or on the banks of rivers, though the moisture be as great as in marshy countries. It is the general opinion that these diseases owe their origin to certain vapours which are given out during the putrefaction of vegetable substances; but of the nature of these miasmata nothing is known. In the West Indies marshes are most fatal to the inhabitants just when they are almost (but not quite) dried up by the heat of the weather. It is then that the exhalations are most abundant and most deadly. It is known that smallpox, &c. may be communicated by the mixture of a particular matter with the blood. It is possible, though not very probable, that this matter may at times exist in the atmosphere in the state of vapour, or in combination with one or other of the constituents of air, most probably the aqueous vapour, and the disease may be communicated by breathing such air. It is a conceivable thing that the matter, which, like a ferment, is capable of inducing certain diseases when it enters into the blood, may exist occasionally united to the aqueous vapour in the atmosphere.

It has been ascertained, by experiments that seem conclusive, that these noxious states of the atmosphere, however they originate, may be destroyed, and the air rendered healthy, by mixing it with chlorine. This is most easily accomplished by introducing into the chamber to be so purified a quantity of chloride of lime in an open dish and pouring on it sulphuric acid. The chlorine is disengaged and speedily fills the apartment. Then the mixture is withdrawn and the room ventilated. See Climate, Meteorology, Pneumatics. (l.)