COLD.

It is often disputed whether cold has any actual existence, or should be considered as merely the privation of heat. Nor is that question of a modern date. Plutarch attempted to discuss it, in his tract De primo frigido ; and the reasonings which he there employs, though abundantly vague, are yet curious. Cold, he says, affects the senses as well as heat; and it is not less active, since it condenses and consolidates bodies. He therefore inclines to the opinion, that cold is a distinct and independent power in nature. With the Stoic philosophers, he regards air as by its constitution cold and dark; and hence water drawn from a well freezes on being exposed to the atmosphere, while rivers overshadowed by high banks seldom freeze, and even where their surface congeals, the heat is not exhaled, but only driven down nearer the bottom.

It is contrary to sound physics to admit more principles than are indispensably required, and this argument alone may be sufficient for the rejection of cold as a distinct power in nature. What we term cold, in reference to our feelings, is merely the diminution of heat; but the existence and materiality of heat rest on a very different foundation. The introduction of heat into a body is accompanied by the infusion of a certain extrinsic repulsive force, and its passage through the mass is connected with a series of depending internal motions, which imply the regular expenditure of time and velocity. The contraction which follows on the diminution of heat is due to the mutual attractive powers of the particles of the substratum, now exerted with less opposition. That expansion, again, which some fluids manifest in the act of congelation, proceeds from the operation of the principle of crystallization, with the recondite nature of which we are still unacquainted.

The notion that cold has a separate and independent existence, appears, however, to receive some countenance from the elegant experiment of collecting and concentrating the frigorifie impressions in the focus of a metallic reflector. This curious fact, which is one of the oldest in physical science, has lately been revived, and combined with circumstances of peculiar interest. The experiment was first mentioned about the year 1590, by Baptista Porta, in the enlarged edition of his Magia Naturalis, when the four books of which it originally consisted were augmented to twenty, at the very time that his ingenious countryman Sanctorio had invented and applied to medical purposes the air thermometer. Porta relates, that, if a shut eye be held in the focus of a speculum, before which is placed a ball of snow, intense cold will be felt on the eye-lid.^[1. Si quis candelam in loco, ubi spectabilis res locari debet, apposuerit, accedet candela per aërem usque ad oculos, ut iLos calore, et lumine offendet; hoc autem mirabilius erit, ut calor, ita frigus reflectitur, si eo loco nix objiciatur, si oculum retigerit, quia sensibilis, etiam frigus percipiet. ] Cavalieri, the celebrated discoverer of infinitesimals, in his work on the conic sections, printed in 1632, and entitled Lo Specchw Ustorio, extended the experiment to all impressions which he conceived to be propagated in straight lines, not only to those of heat and cold, but to those of sound, and even smell.^[2. Cavalieri mentions that, with a spherical speculum made of lead, and indifferently polished, he was able to inflame dry substances by the reflexion of a charςoal fire; and that, with a deep truncated parabolic speculum, badly polished, he produced the same effect in the open focus, from a small fire of wood at the distance of five feet. Esperienza di questo ho fatto io, che con vno Specchio sfericodi piõbo ancor mal polito, ho acceso il fuoco nella materia arida al fuoco di carboni; e di più ľhò fatto con la superficie parabolica, cioè con vn canone parabolico, che hauea il suo foco vicino alla cima, essendo esso specchio parabolico trôcato pur nella cima, quai’ era di stagno, e mal polito i tal che opponendolo al fuoco, ò alla fiamma di ben poca legna, nella distanza di tre braccia, ponendo la mano li, dou’era la parte trôcato, et il foco della parabola, non vi si potea sostenere, anzi vi s’accese fuoco; la quai cosa potria alcuno applicare al riscaldameto delle stanze, ò alle distillationi. Pp. 85, 86.—In general, says this ingenious mathematician and philosopher, the same form of speculum which concentrates light and heat must likewise collect cold, which spreads.from its source, from a mass of snow for instance, in straight lines. The hyperbola is, therefore, the figure which he thinks the best adapted for the purpose; and he proposes this for condensing the smells radiated from an odoriferous substance. Hora dunque basterà quello, che si è detto di sopra intorno al lume, e calore, potendo noi néĩľistesso tempo intendere le medesime cose aneo per il freddo, che dilatadosi dal eorpo freddo ad ognii positione per linea retta, e perciò nelľ infinite lince, che si partono dal eorpo freddo, come dalla neue, essendoui dentro le parallele, che sono vnite dallo specchio parabolico, e le diuergenti, che sono vnite dall’ elittico e le conuergenti vnite dall’ iperbolico, con opporre alcun di questi specclii ad vna massa di neue, ò di ghiacco, sentiremo nel loro foco essere il freddo fatto molto gagliardo, ma per questo effetto sarà più atto l’iperbolico di tutti, come quello, che raccoglierà maggior quantité di lince fredde; e questo basti ancora circa il freddo, potendosi forsi in vn certo modo creder, che talle effetto accadesse anco intorno à gli odori, pronando noi dilatarsi pur quelli dalli eorpi odoriferi verso ogni lauda. Id. p. 128. ] It was afterwards frequently repeated at Florence, by the Academy del Cimento, with the important addition of the thermometer, which that learned body had the merit of introducing into practice. Similar experiments were next performed by Mariotte in France.

Specula and burning glasses appear in the sequel to [7:7:58]have been allowed to fall into great neglect. We find scarcely any mention of their application to physical researches, till, after the lapse of more than half a century, Kraft repeated, at St Petersburg, during the severe winter of 1740, the frigorifie experiment of the Italian philosophers, with a reflector belonging to the cabinet of the Imperial Academy. Ambitious to operate on a grand scale, he selected three huge blocks of clear ice, nearly of a cubical form, each side being two, four, and five feet; but, to save the trouble of transporting them, he carried the speculum out of doors. No sensible effect, however, was then perceived by him, though he used the air thermometer on account of its extreme delicacy. In 1744, this academician again resumed the observation, and with scarcely better success, having obtained only a doubtful cold of three degrees. The cause of the failure was evidently his performing the experiment out of doors, and not in a warm room. The blocks of ice had, by long standing, acquired almost the same temperature as their ambient medium. Had the air happened to become suddenly colder, they might, from their relative condition, have excited impressions even of heat, and thus have perplexed philosophy for many years afterwards.

Such unsatisfactory results, from the action of a mass of ice of above a ton weight, seem for a long time to have shaken the belief in former experiments; and the subject was almost forgotten, when Pictet of Geneva, in 1781, repeated the original observation, on a small scale, indeed, but with entire success. Since that time a pair of brass reflectors, with a wire case for holding charcoal or snow, has been deemed an essential apparatus in every physical cabinet. The concentration of cold in the focus of a speculum always excites surprise; and the experiment is often exhibited with a sort of mysterious air, as if it established, or at least rendered probable, the distinct and material existence of cold; but, in fact, it is not more difficult to conceive the impressions of cold to be collected than those of heat. Both those impressions are only relative to the temperature of the atmosphere, which serves as the medium of their transmission. The one process terminates with the deposition of a portion of heat, the other with its abstraction.

The diminution of heat, or the increase of cold, is produced in nature under four different circumstances: 1. By the obliquity or absence of the sun; 2. by the tenuity of the higher atmosphere; 3. by the evaporation which takes place in dry air; and, 4. by the chilling impression shot downwards from a clear and serene sky.

I. In our temperate climates the thermometer in winter very seldom descends fifteen degrees on Fahrenheit’s scale, below the point of congelation; but in the higher latitudes the intensity of the cold is often far greater. In the northern parts of Sweden and Russia, the rivers and ordinary lakes are frozen to the depth of several feet; wine, and even ardent spirits, become converted into a spongy mass of ice, and, as the cold still augments, it penetrates the living forests, and congeals the very sap of the trees, which occasionally burst from this internal expansion with tremendous noise. The Baltic Sea has been repeatedly covered with a solid floor of ice, capable of transporting whole armies, with all their stores and engines of war. Those waters, indeed, arc only brackish; but the more northern ocean itself has often been frozen to a very considerable thickness. In Siberia and Hudson’s Bay, and even in the northern provinces of Sweden, mercury has been at some times observed contracted by exposure into a solid semi-metal; and, consequently, the cold which then prevailed must have exceeded seventy-one degrees, or thirty-nine below the commencement of Fahrenheit’s scale.

2. In elevated tracts the increase of cold is very striking. Even at an altitude of three miles and a half, the air is generally sixty-eight degrees colder than at the level of the sea. On the summit of the Andes, therefore, a' thermometer would often sink perhaps under the beginning of Fahrenheit’s scale; and it seems probable that mercury would naturally freeze in winter on the top of Mont Blanc. Mountains are hence regarded as the grand stores or depositaries of cold in the milder climates. In every country, therefore, the air of subterraneous caves, and the water of deep springs or wells, are during the summer months comparatively cold. Hence the obvious advantage of cellars, in addition to their preserving an uniform temperature, which is so favourable to the ripening of the liquors deposited in them. But the air at the bottom of an open and very deep pit must be colder than the mean state of the ground; for in all the changes which take place at the surface, the cold air will descend, and the warm air still float over the mouth of the pit. The wealthier classes of antiquity were accustomed, accordingly, to cool the wine for their tables by suspending it for some time in a bucket let down near the surface of profound wells.

3. Evaporation is a natural process, by which heat is powerfully abstracted by the exhaling moisture, while this assumes a gaseous constitution in the act of combining with dry air. The fact seems to have been known in the warm regions of the East at a very early period of society, suggested probably by the familiar use of a rude unglazed pottery for all culinary purposes. The Egyptians, and other inhabitants of the sultry shores of the Levant, have, from the remotest ages, cooled the water for drinking in their porous jars. Athenaeus reports, from a history of Protagorides, that King Antiochus had always a provision for his table prepared in that way. The water having been carefully decanted from its sediment into earthen pitchers (iιδgs∕ais ×egaμs∕a∕ζ), these were transported to the highest part of his palace, and exposed to the clear and keen atmosphere (sξa∕0g∕aξou<r∕v), two boys being appointed to watch them the whole night, and kept constantly wetting their sides. This labour of sprinkling the surface of the jars seems to have been afterwards spared, in consequence probably of the adoption of a more porous kind of earthenware. Galen, in his Commentary on Hippocrates, relates, that he witnessed the mode of cooling water, which was practised in his time, not only at Alexandria, but over all Egypt. The water having been previously boiled, was poured at sunset into shallow pans (αyyo∕ς ũōrgaMivoiç), which were then carried to the house-tops, and there exposed during the whole night to the wind; and, to preserve the cold thus acquired, the pans were removed at day-break, and placed on the shaded ground, surrounded by leaves of trees, prunings of vines, lettuce, or other slow-conducting substances.

The bottles or bags made of goat-skins, in which the wandering Arabs are wont to carry their scanty provision of water, allowing a small portion of the liquid to transude and exhale, render it by consequence comparatively cool, and better fitted to mitigate or allay the intolerable thirst created in traversing their sandy deserts. In Guinea, it is customary to fill gourds or calabashes with water, and suspend them all night from the outer branches of trees.

The Moors introduced into Spain a sort of unglazed earthen jugs, named bucaros or alcarrazas, which, being filled with water, present to the atmosphere a surface constantly humid, and furnish by evaporation, during the dry and hot weather, a refreshing beverage. The same practice has been adopted by degrees in various parts of the south of Europe. In India, during certain months, the apartments are kept comparatively cool, by dashing [7:7:59]water against the matting of reeds or bamboos which line the doors and outside of the walls. Even the more luxurious mariners, in their voyages between the tropics, are accustomed to cool their wines by lapping the bottle with wet flannel, and suspending it from the yard or under the cabin windows. In all such cases the effect is accelerated, though not augmented, by the swiftness of the current of air. What have been called Egyptian coolers, and lately produced by our potters, are less perfect in their operation. Being very thick, they require only to be soaked in water, and the evaporation from their surface cools the adjacent air. On the inside, however, where the bottle is placed, the action, in consequence of the confined humidity, must be enfeebled. In damp weather, these vessels, it is evident, are entirely useless.

The natives of India likewise are enabled, by directing a skilful process of evaporation, to procure for themselves a supply of ice during their short winter. In the upper country, not far however from Calcutta, a large open plain being selected, three or four excavations are made in it, about thirty feet square and two feet deep, and the bottom covered to the thickness of nearly a foot with sugar canes or dried stalks of Indian corn. On this bed are placed rows of small unglazed earthen pans, about an inch and quarter deep, and extremely porous. In the dusk of the evening, during the months of December, January, and February, these are filled with soft water previously boiled and suffered to cool; when the weather is very fine and clear a great part of the water becomes frozen during the night. The pans arc regularly visited at sunrise, and their contents thrown into baskets which retain the ice. These are now carried to a conservatory, made by sinking a pit fourteen or fifteen feet deep, lined with straw under a layer of coarse blanketing. The small sheets of ice are thrown down into the cavity, and rammed into a solid mass. The mouth of the pic is then closed up with straw and blankets, and sheltered by a thatched roof.

4. It was stated in the article Climate, that impressions of cold are constantly showered down from a clear and azure sky. These effects are no doubt more conspicuous in the finer regions of the globe. Accordingly, they did not escape the observation of the ancients, but gave rise to opinions which were embodied in the language of poetry. The term ar i % was applied only to the grosser part of the atmosphere, while the highest portion of it, free from clouds and vapour, and bordering on the pure fields of aether, received the kindred appellation of aièpa. But this word and its derivatives have always been associated with ideas of cold. We have seen that the verb sξα∕- őg/αζω is used by Athenaeus to signify the cooling of a body by exposure under a serene sky. Homer uses the term α∕0goς, in speaking of the reception of his hero, when overcome with cold and toil.^[3. A,ιS1tf xa,ι xaμaτa> ⅛tδfLHfL'tiov ∏γtv is oι×av∙ Odyss. lib· xiv. 318. ] The same poet of nature applies the epithet aι∂ξηγivr ∣ ζ or aι∂oηγiViT7iζ, or frigorifie, to Boreas, the north wind.^[4. '∩S 5’ in τ<za(psja∕ ιι<1>aiis ∆ms ιχτοτιονται, Ψ⅛j^a∕ vnccι pιm s aι^ζf γιvios IIoļiao. Iliad· lib· XIX. 357—8. Ka Boζi>ιs aι6ζ> γε ∣∣ ιτns> pιγa xvua xuaιvδωv. Odyss, lib. V. 296. ] The chorus in the Antigone of Sophocles deprecates the pelting storm, and likewise the cold (α∕^∕α) of inhospitable frozen tracts.^[5. ⅛υa ’ aυλωv tfaγωv aιSya xoa Şutroftßļa φiuγiw βιλ∏∙ Antigone, 357∙ ] The word ct/^/o¢ is employed by Herodotus to signify a chill as well as a dry atmosphere.^[6. Qi1y,on1oy γα.% ⅛ irrt τα vSa o τt s τs aι(ζ as xxι τns ĩļoeav. Euterpe. ] Of the same import is the expression in Horace, sub Jove frigido.

In the finer climates, especially, a transpiercing cold is, therefore, felt at night under the clear and sparkling canopy of heaven. The natives carefully avoid exposing themselves to this supposed celestial influence; yet a thin shed of palm leaves may be sufficient at once to scrcen them from the scorching rays of the sun, and to shelter them against the chilling impressions rained from the higher atmosphere. The captains of the French gallies in the Mediterranean used formerly to cool their wines in summer by hanging the flasks all night from the masts. At daybreak they were taken down and lapped in several folds of flannel, to preserve them in the same state. The frigorifie impression of a serene and azure sky must undoubtedly have concurred with the power of evaporation in augmenting the energy of the process of nocturnal cooling practised anciently in Egypt, and now systematically pursued in the higher grounds of India. As the chilness accumulated in the ground is greatest in clear nights, when the moon shines brightest, it seemed very natural to impute this effect partly to some influence emanating from that feeble luminary. It was long imagined that the lunar beams are essentially cold; and some philosophers, at no remote period, have attempted even to prove the fact by experiment. Mr Boyle, though he rejected judicial astrology, was yet disposed to admit the notion of stellar influences.

The obvious mode of cooling water, or other liquids, by the infusion of ice or snow, was practised in the warmer countries from the earliest ages. It is even mentioned in the Proverbs of Solomon, “ As the cold of snoiv in the time of harvest, so is a faithful messenger to them that send him.” Aristotle, presuming that the finer parts of water are dissipated by congelation, maintained that it is pernicious to drink melted snow. This speculative opinion seems not, however, to have been regarded by the ancients. Theocritus calls snow-water an ambrosial drink, noroς aμβξoff ∣ oς. Xenophon mentions the practice of cooling wine by the addition of snow. It is related by the historians of Alexander the Great, that in his Indian expedition, when he laid siege to the city of Petra, he commanded thirty pits to be dug and filled with snow, which was covered over with oak branches. The luxurious Romans had excavations regularly formed for keeping snow the whole year, chaff and other light substances being employed to preclude from it the access of heat. But as the snow preserved in this way could not escape being soiled, instead of mixing it directly in the drinking cup, a more refined practice was introduced of surrounding the silver goblet which contained the liquor with a mass of the melting snow. This improvement was ascribed to the profligate emperor Nero. Similar modes of storing up snow have been adopted in all the warm countries. The caves on the sides of Mount Aetna are considered as natural magazines for supplying a material which is not only carried down to Palermo and Messina, but even shipped to the island of Malta. The Italians formerly cooled their wine by setting the large glass flasks containing it in wide vessels of wood or cork, the intervening space being filled with snow on which water was poured.

Saltpetre or nitre being almost a natural production of the East, its property of rendering water cold by solution was probably known from a very remote period to the oriental nations. This process of cooling is described in the Institutes of Akbar as the discovery of that enlightened prince, who governed India with parental mildness from the year 1560 to 1605. One part of nitre is directed to be thrown into a vessel containing two parts of water; and a gugglet of pewter or silver filled with pure water, [7:7:60]and having its mouth close stopped, is then stirred quickly in the mixture for the space of a quarter of an hour.

The frigorifie property of nitre was probably first communicated from India or Persia to Europe, and seems to have become known to the Italians about the middle of the sixteenth century. As early as the year 1550, all the rich families in Rome cooled the liquors for their tables by dissolving that salt in water. Into a vessel of cold water the nitre was gradually added in the proportion of a fourth or fifth part, while a globular bottle with a long neck, containing the wine or water to be cooled, was whirled rapidly round its axis. The salt, being afterwards recovered by crystallization, would always serve the same purpose again with undiminished effect. In India every family of distinction keeps a domestic, whose sole employment is to cool liquors by this process; but nitre being cheap in that country, it is used in larger proportions, and the water charged with it is allowed to become a perquisite of the operator.

The application of salts to produce cold was extended by Boyle, and afterwards more successfully by Fahrenheit. But within these twenty years Mr Walker of Oxford, and Professor Lowitz of St Petersburg, have resumed the subject, and produced compound saline powders possessed of intense frigorifie power. The solution of salts in water expanding that liquid, augments its capacity for heat, and consequently depresses its temperature. This effect is likewise the greater in proportion to the quantity of saline matter which can be dissolved. But after water is saturated with one species of salt, it can still absorb some portion of another. Hence the frigorifie effects of solution are always increased by employing a compound dry powder. Nitre and sal-ammoniac, or the nitrate of potash and the muriate of soda, in equal parts, added in the form of a dry powder to three parts by weight of water, will sink Fahrenheit’s thermometer forty degrees. But equal parts of the muriate of ammonia and of the nitrate of potash, with one part and a half of the sulphate of soda or common Glauber’s salt, will cool down three parts of water forty-six degrees. A still greater effect, amounting to fifty-seven degrees, is produced by dissolving equal parts of the nitrate of ammonia and of the carbonate of soda in one part of water. The frigorifie action is in general augmented by throwing the desiccated powder into dilute acid instead of water. Thus three parts of the phosphate of soda, and two parts of the nitrate of ammonia, joined to rather more than one part of weak nitric acid, will sink the thermometer seventy-one degrees.

These changes induced on the temperature of the liquid menstruum are no doubt considerable, yet they are still only transient, and the process requires some address and manipulation, not always readily attained. But the principle of evaporation, when rightly understood, leads to a far easier mode of cooling liquids, which maybe prolonged at pleasure. A close investigation of that principle, at the very commencement of his philosophical labours, has conducted Professor Leslie through the whole train of his discoveries on the subject of refrigeration. The process of evaporation had not then been examined with attention. The depression of temperature which always accompanies it was hastily supposed to be proportiònal to the rate with which the moisture is dissipated, and to be therefore augmented by every circumstance that can accelerate this effect. If water, contained in à porous vessel, expose on all sides its surface to a current of air, it will cool down to a certain point, and there its temperature will remain stationary. The rapidity of the current must no doubt hasten the period of equilibrium, but the degree of cold thus induced will be still the same. A little reflection may discover how this happens. Though the humid surface has ceased to grow colder, the dispersion of invisible vapour, and the corresponding abstraction of heat, still continues without intermission. The same medium, therefore, which transports the vapour, must also furnish the portion of heat required for its incessant formation. In fact, after the water has been once cooled down, each portion of the ambient air which comes to touch the evaporating surface must, from its contact with a substance so greatly denser than itself, be likewise cooled to the same standard, and must hence communicate to the liquid its surplus share of heat, or the difference between the prior and the subsequent state of the solvent, which is proportional to the diminution of temperature it has suffered. Every shell of air which encircles in succession the humid mass, while it absorbs, along with the moisture which it dissolves, the measure of beat necessary to convert this into steam, does at the same instant thus deposit an equal measure of its own heat on the chill exhaling surface. The abstraction of heat by vaporization on the one hand, and, on the other, its deposition at the surface of contact, are therefore opposite contemporaneous acts, which soon produce a mutual balance, and thereafter the temperature induced continues without the smallest alteration. A rapid circulation of the evaporating medium may quicken the operation of those causes; but so long as it possesses the same drying quality, it cannot in any degree derange the resulting temperature. The heat deposited by the air on the humid surface becomes thus an accurate measure of the heat spent in vaporizing the portion of moisture required for the saturation of that solvent at its lowered temperature» The dryness of the air is therefore, under all circumstances, precisely indicated by the depression of temperature produced on a humid surface which has been exposed freely to its action.

Guided by these views, Mr Leslie was enabled to construct a correct hygrometer, that should indicate the dryness of the air, from the diminution of temperature which a small body of water, exposed on all sides, suffers by evaporation. His efforts again to improve this instrument led him next to the invention of the differential thermometer, which was converted into an hygrometer by having one of its balls covered with cambric, lint, or tissue paper, capable of being easily wetted. Reduced to such a delicate and commodious form, it detected with the utmost precision, and under all circumstances, the relative condition of the air in regard to dryness.

It appears that absorbent substances, exposing a broad surface, are capable of assimilating to their previous state the air confined over them. Flannel, for instance, which has been intensely dried, will support a remarkable degree of dryness in a close receiver. The trap-rock and compound clays, brayed into a coarse powder, and desiccated before a strong fire, will exert a more powerful and extended action. But dried oat-meal will act with equal energy, and for a longer time. Of the saline substance⅛, the muriate of lime absorbs moisture with the greatest and most protracted force. After it has become drenched with humidity, it may likewise be recovered again, though the process of restoring it unaltered is rather troublesome. But the best and most powerful absorbent is the concentrated sulphuric acid, or the oil of vitriol of commerce, which continues for a long time to attract moisture with almost undiminished force, and possesses besides the valuable property, after it has become charged with humidity, of being easily restored again by the application of heat to its original strength.

To cool water in any climate or state of the atmosphere, we have only therefore to put it into a small porous vessel, [7:7:61]presenting on all sides a humid surface, and to suspend this within a close wide cistern, of which the bottom is covered with a layer of sulphuric acid. The broad surface of the acid absorbing the moisture as fast as it diffuses itself through the confined air, keeps that medium constantly at a point of extreme dryness, and thus enables it to support, with undiminished vigour, the process of evaporation.

In practice, the cistern or refrigeratory, having a broad cylindrical form, from twelve to sixteen inches in diameter, and composed of dense well-glazed earthenware (see Plate CLXXI∏∙ fig· 7), is placed in a cellar or other cool place, and charged with sulphuric acid to the height of about half an inch from the bottom. One of the porous earthen pots, being filled up to the lip with water fresh drawn from the well, is set upon a low porcelain stand in the middle of the cistern, to which the lid or cover is then carefully adapted. In the space of from three to perhaps five hours, the cooling is nearly completed, and the pot should now be removed; for though the water will be kept at the same degree of coldness as long as it remains shut up within the refrigeratory, the acid would be unnecessarily weakened by the incessant absorption of moisture.

The production of cold is greater when the cistern is large, or when a small pot is used, insomuch that the effect will be diminished one half if the humid surface should equal that of the acid, the opposite actions of such surfaces inducing an exactly intermediate state with respect to dryness and moisture in the condition of the aerial medium. The power of evaporation is also diminished in the low temperatures. Thus, if the atmosphere were at 95° by Fahrenheit, the water within the refrigeratory might be cooled down 36°, or brought to 59°; but if the thermometer be at 50°, the water can be cooled only 18°, which brings it to the freezing point. This seems to be a very convenient property, since the power of the refrigeratory is always the greatest at the season when its application is most wanted.

It is easy, therefore, by such means to cool water in our climate at all times to near the freezing point, and, even under the torrid zone, to reduce it to the temperature of 60°, which in those regions is sufficient perhaps for essential comfort.

By supplying a succession of porous earthen pots, the acid will continue to act with scarcely diminished force, till it has absorbed half its weight of moisture; during which time it will have assisted in cooling about fifty times that quantity of the water exposed to evaporation. At this stage the dilute acid should be drawn off, and a charge of concentrated acid again introduced into the refrigeratory· . ....

r - This method of procuring cold, it will readily be perceived, could be employed with advantage for various domestic purposes. For instance, butter may in summer be kept cool for the table, by putting it, after being washed with water, into a wet porous pot, and shutting this up for a couple of hours in the refrigeratory. To cool wine sufficiently, one bottle only is used at a time in the smallest refrigeratory. A sheath of stocking or flannel, previously soaked in water, being drawn over the body of the bottle, it is laid in a reclined position on one of the porcelain sliders, near the surface of the acid, and allowed to remain shut up during the space of three or four hours.

The refrigerating combination here employed produces its effect, by a sort of invisible distillation carried on by the play and circulation of the included air. The minute portions of moisture which successively combine with the contiguous medium, must abstract from the mass of water as much heat as would support them in the state of vapour, or would in ordinary cases convert them into steam. This vapour again being conveyed through the air, is attracted by the sulphuric acid, and, recovering its liquid constitution, deposits the heat which it had borne away. The acid is therefore warmed at the expense of the water subjected ∙to evaporation, and the whole performance of the apparatus consists in a mere transfer and interchange of condition.