# Theory of Heat (Maxwell, 4th edition)/Chapter 1

A TREATISE
ON
H E A T.
CHAPTER I.
I N T R O D U C T I O N.
The distinction between hot bodies and cold ones is familiar to all, and is associated in our minds with the difference of the sensations.which we experience in touching various substances, according as they are hot or cold. The intensity of these sensations is susceptible of degrees, so that we may estimate one body to be hotter or colder than another by the touch. The words hot, warm, cool, cold, are associated in our minds with a series of sensations which we suppose to indicate a corresponding series of states of an object with respect to heat.
We use these words, therefore, as the names of these states of the object, or, in scientific language, they are the names of Temperatures, the word hot indicating a high temperature, cold a low temperature, and the intermediate terms intermediate temperatures, while the word temperature itself is a general term intended to apply to any one of these states of the object.
Since the state of a body may vary continuously from cold to hot, we must admit the existence of an indefinite number of intermediate states, which we call intermediate ​temperatures. We may give names to any number of particular degrees of temperature, and express any other temperature by its relative place among these degrees.
The temperature of a body, therefore, is a quantity which indicates how hot or how cold the body is.
When we say that the temperature of one body is higher or lower than that of another, we mean that the first body is hotter or colder than the second, but we also imply that we refer the state of both bodies to a certain scale of temperatures. By the use, therefore, of the word temperature, we fix in our minds the conviction that it is possible, not only to feel, but to measure, how hot a body is.
Words of this kind, which express the same things as the words of primitive language, but express them in a way susceptible of accurate numerical statement, are called scientific terms, because they contribute to the growth of science.
We might suppose that a person who has carefully cultivated his senses would be able by simply touching an object to assign its place in a scale of temperatures, but it is found by experiment that the estimate formed of temperature by the touch depends upon a great variety of circumstances, some of these relating to the texture or consistency of the object, and some to the temperature of the hand or the state of health of the person who makes the estimate.
For instance, if the temperature of a piece of wood were the same as that of a piece of iron, and much higher than that of the hand, we should estimate the iron to be hotter than the wood, because it parts with its heat more readily to the hand, whereas if their temperatures were equal, and much lower than that of the hand, we should estimate the iron to be colder than the wood.
There is another common experiment, in which we place one hand in hot water and the other in cold for a sufficient ​time. If we then dip both hands in the same basin of lukewarm water alternately, or even at once, it will appear cold to the warmed hand and hot to the cooled hand.
In fact, our sensations of every kind depend upon so many variable conditions, that for all scientific purposes we prefer to form our estimate of the state of bodies from their observed action on some apparatus whose conditions are more simple and less variable than those of our own senses.
The properties of most substances vary when their temperature varies. Some of these variations are abrupt, and serve to indicate particular temperatures as points of reference; others are continuous, and serve to measure other temperatures by comparison with the temperatures of reference.
For instance, the temperature at which ice melts is found to be always the same under ordinary circumstances, though, as we shall see, it is slightly altered by change of pressure. The temperature of steam which issues from boiling water is also constant when the pressure is constant.
These two phenomena therefore—the melting of ice and the boiling of water—indicate in a visible manner two temperatures which we may use as points of reference, the position of which depends on the properties of water and not on the conditions of our senses.
Other changes of state which take place at temperatures more or less definite, such as the melting of wax or of lead, and the boiling of liquids of definite composition, are occasionally used to indicate when these temperatures are attained, but the melting of ice and the boiling of pure water at a standard pressure remain the most important temperatures of reference in modern science.
These phenomena of change of state serve to indicate only a certain number of particular temperatures. In order to measure temperatures in general, we must avail ourselves of some property of a substance which alters continuously with the temperature.
​The volume of most substances increases continuously as the temperature rises, the pressure remaining constant.There are exceptions to this rule, and the dilatations of different substances are not in general in the same proportion; but any substance in which an increase of temperature, however small, produces an increase of volume may be used to indicate changes of temperature.
For instance, mercury and glass both expand when heated, but the dilatation of mercury is greater than that of glass. Hence if a cold glass vessel be filled with cold mercury, and if the vessel and the mercury in it are then equally heated, the glass vessel will expand, but the mercury will expand more, so that the vessel will no longer contain the mercury. If the vessel be provided with a long neck, the mercury forced out of the vessel will rise in the neck, and if the neck is a narrow tube finely graduated, the amount of mercury forced out of the vessel may be accurately measured.
This is the principle of the common mercurial thermometer, the construction of which will be afterwards more minutely described. At present we consider it simply as an instrument the indications of which vary when the temperature varies, but are always the same when the temperature of the instrument is the same.
THE MERCURIAL THERMOMETER.
This thermometer consists of a glass tube terminating in a bulb, the bulb and part of the tube being filled with mercury, and the rest of the tube being empty. We shall suppose the tube to be graduated in any manner so that the height of the mercury in the tube may be observed and recorded. We shall not, however, assume either that the tube is of uniform section or that the degrees are of equal size, so that the scale of this primitive thermometer must be regarded as completely arbitrary. By means of our thermometer we can ascertain whether one temperature is higher or lower than another, or equal to it, but we cannot assert that the difference between two temperatures, a and b, is greater or less than the difference between two other temperatures, c and d.
We shall suppose that in every observation the temperature of the mercury and the glass is equal and uniform over the whole thermometer. The reading of the scale will then depend on the temperature of the thermometer, and, since we have not yet established any more perfect thermometric scale, we shall call this reading provisionally 'the temperature by the arbitrary scale of the thermometer.'
The reading of a thermometer indicates primarily its own temperature, but if we bring the thermometer into intimate contact with another substance, as for instance if we plunge it into a liquid for a sufficient time, we find that the reading of the thermometer becomes higher or lower according as the liquid is hotter or colder than the thermometer, and that if we leave the thermometer in contact with the substance for a sufficient time the reading becomes stationary. Let us call this ultimate reading 'the temperature of the substance.' We shall find as we go on that we have a right to do so.
Let us now take a vessel of water which we shall suppose to be at the temperature of the air, so that if left to itself it ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/22 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/23 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/24 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/25 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/26 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/27 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/28 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/29 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/30 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/31 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/32 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/33 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/34 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/35 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/36 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/37 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/38 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/39 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/40 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/41 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/42 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/43 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/44 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/45 ​Page:Theory of Heat, James Clerk Maxwell, Fourth Edition.djvu/46 ​It appears, therefore, that gases are distinguished from other forms of matter, not only by their power of indefinite expansion so as to fill any vessel, however large, and by the great effect which heat has in dilating them, but by the uniformity and simplicity of the laws which regulate these changes. In the solid and liquid states the effect of a given change of pressure or of temperature in changing the volume of the body is different for every different substance. On the other hand, if we take equal volumes of any two gases, measured at the same temperature and pressure, their volumes will remain equal if we afterwards bring them both to any other temperature and pressure, and this although the two gases differ altogether in chemical nature and in density, provided they are both in the perfectly gaseous condition.
This is only one of many remarkable properties which point out the gaseous state of matter as that in which its physical properties are least complicated.
In our description of the physical properties of bodies as related to heat we have begun with solid bodies, as those which we can most easily handle, and have gone on to liquids, which we can keep in open vessels, and have now come to gases, which will escape from open vessels, and which are generally invisible. This is the order which is most natural in our first study of these different states. But as soon as we have been made familiar with the most prominent features of these different conditions of matter, the most scientific course of study is in the reverse order, beginning with gases, on account of the greater simplicity of their laws, then advancing to liquids, the more complex laws of which are much more imperfectly known, and concluding with the little that has been hitherto discovered about the constitution of solid bodies.