INTRODUCTION.

1. The word 'Heat' is used in the following treatise to denote that agent which produces a certain well-known sensation when applied to the human body.

2. But this sensation while so familiar to us that every one must immediately recognise what agent is meant when the term Heat is used, is yet not sufficiently definite or constant to afford us the means of accurately measuring the amount of heat which a body possesses. The same substance may at the same time appear warm to one individual and cold to another; nay, it may even be pronounced warm by one of our hands and cold by the other. Our judgment with regard to the amount of heat possessed by the substance in question is thus found to depend upon the state of our bodies, and as this changes from time to time we cannot therefore make use of our sensations as a means of measuring heat.

3. In the study of this subject it is thus of primary importance to become acquainted with some instrument by the aid of which the state of bodies with regard to heat may be accurately determined. The Thermometer fulfils this requirement, and a description of it will therefore form the commencement of this treatise.

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4. When the Thermometer has been described the various effects produced in bodies by the presence of heat may next with advantage be studied. Such effects are change of volume; of condition; of hardness; chemical change; with other effects which will afterwards be mentioned.

5. The laws which regulate the distribution of heat will next be considered, and under this head it will be necessary to recognise two distinct modes of conveyance of this agent from one body to another. Every one is familiar with the

body with a hot one, whether

fact that the contact of a cold solid, liquid, or gaseous, will cause the latter to part with some of its heat. But this is not the only way by which heat may reach us; for we derive a very considerable amount from the sun, although this luminary is at a great distance from the earth, while the intervening space does not contain that gross form of matter which serves to convey heat by

contact.

We are thus taught that a hot body parts with its heat in two ways,―

(1) By contact with a cold body;

(2) By radiation through space.

And we believe, moreover, that radiant heat traverses space with the enormous velocity of 190,000 miles per second.

6. We have thus the means of making a very convenient and easily perceived classification of the modes in which Heat exhibits itself to us.

There is, in the first place, what we may call absorbed heat, which resides in a hot body, and often remains in it for a considerable time; and we have, in the next place, radiant heat, which is heat in the act of passing through space with a very great velocity.

7. In treating of the laws which regulate the distribution of heat this distinction may with propriety be observed, and

this part of the subject will thus divide itself into two. It will be desirable to consider, in the first place, radiant heat-its nature, and the laws which regulate its distribution; and then to consider the laws which regulate the distribution of absorbed heat, while under this last heading the capacity of bodies for heat will be brought forward.

8. In treating of radiant heat theoretical views will for the first time be introduced. It is well known that there have been two distinct theories regarding the nature of Heat. In one of these it is viewed as a substance which insinuates itself between the particles of a hot body, while in the other it is regarded as a species of motion taking place amongst these particles. Such views could not well be introduced at an earlier stage of the work, since the more prominent effects of heat upon bodies form a set of phenomena that are capable of being explained tolerably well by either hypothesis. They are therefore somewhat unsuitable as tests of a theory, while, on the other hand, they are of extreme importance as facts. But the study of radiant heat enables us to pronounce with a near approach to certainty that this influence is not a substance ejected from a hot body, but rather a description of undulatory motion transmitted through a medium pervading all space.

9. It would appear to follow, as a corollary from this view, that ordinary heat into which radiant heat is transformed when absorbed must also be a species of motion, and this idea will be abundantly confirmed in the next part of the subject, where the various sources of heat will be treated of, and the nature of this agent as well as its connexion with other properties of matter fully discussed.

10. Lastly, this treatise will contain throughout notices of some of the most important practical applications of the laws of Heat, and also of certain terrestrial and cosmical adaptations in which these laws play a very important part.

It will be observed throughout that the term Caloric is avoided as much as possible, since this term has come to be associated with that theory which regards heat as a substance and not as a species of motion.

BOOK I.

EFFECTS OF HEAT UPON BODIES.

CHAPTER I.

Temperature, and its measurement by Thermometers.

DEFINITION OF TEMPERATURE.

11. The temperature of a body may be defined to be its state with respect to sensible heat. When the amount of sensible heat in a body increases, its temperature is said to rise, and when this diminishes its temperature is said to fall. It is expedient to discuss at the outset certain fundamental principles which underlie all measurements of temperature.

12. If there be two substances, as for instance water and mercury, in such a condition that when brought intimately into contact with one another and shaken together neither of them changes its state with respect to heat, then these two bodies may be said to be in a state of equilibrium of temperature with each other. Suppose now that the mercury is in a similar state with respect to a third substance, oil. We have thus the water in equilibrium of temperature with the mercury and the mercury in equilibrium of temperature with the oil. Now we know, as the result of experience, that we shall also have the water in equilibrium of temperature with the oil. In fine, if a series of bodies be in equilibrium of temperature with each other in any one order they will be so in any other order; and no matter how they are brought