HEAT is a term which was originally employed to ex press the effects produced by a peculiar condition of bo dies, when they communicated the sensation of warmth. It was also perceived that hot bodies, or those that had the power of communicating warmth, possessed other proper ties, such as that of expanding the substances to which the warmth was imparted, of converting certain solids into liquids, and certain liquids into the state of vapour, and many other operations of great importance in the system of nature. Philosophers soon began to speculate upon the cause of these phenomena, and, with a degree of inaccu racy, to which we are liable in the infancy of our scientific pursuits, they employed the same word to signify both the cause, and the effect produced. It was commonly said, that heat occasioned the warmth and expansion of bodies, and likewise that heat was excited in bodies by the addition of some peculiar kind of matter, or by a certain modification of their pal ticles. The more precise nomenclature of the moderns has tended to correct this error, and has led to the invention of a new term, caloric, to designate the cause, while the word heat is, strictly speaking, only applicable to the effect. As, however, in all the older authors the former phraseology necessarily exists, as it is still adopted in popular language, and as there is no danger of falling into any error, since the distinction has been so fully point ed out, the word heat is frequently employed in its double sense, even by the latest and most correct writers, and it will be used in this way in the following article.
We have already given some account of the nature and effects of caloric under the head of CHEMISTRY; but it is an agent of such extensive importance in the operations of nature,—it produces such powerful effects both upon or ganized and unorganized matter,—it is so intimately con nected with the existence of life, both animal and vegeta ble,—and is so essential to all the processes by which we act upon the bodies around us, when we convert them to our support or utility,—that it well deserves to be farther discussed, and made the subject of a separate article.
The impol tance of the object has produced a consequent share of attention to it from the modern experimentalists; and there is perhaps no one topic on which more curious, and, we may add, more unexpected results :lave been obtained, than have ensued from the re , earches into caloric. The names of Black, Crawford, Itumforcl, Pictet, Gay-Lussac, Prevost, Dalton, and Leslie, many others which will be afterwards referred to, ust suggest the recollection of the many ingenious and elaborate trains of experiments, that have occupied the at tention of philosophers during the last 50 years. It will be to an account of what has been done in this period that we shall principally confine ourselves in the following pages ; for the experiments and hypotheses that were pub lished before this time, are rather to be regarded as curi ous historical records of opinions, than as affording much that is important in the actual advancement of knowledge.
\Ve shall arrange our observations on heat under four heads : 1st, The properties of heat ; 2d, The effects of heat ; 3d, The sources of heat ; and, 4th, The nature of heat. In the course of the article, we shall take an opportunity of tracing the gradual developemcnt of the leading opinions that have successively prevailed on these topics, as well as the most important experiments by which they have been supported.
On the Properties of Heat.
IT might, at first view, appear more regular to begin by investigating the nature of heat, before we described its properties and effects; but it is so difficult to ascertain its nature, and the knowledge which we possess, or rather the conjectures which we form concerning it, are so entire ly derived from the observations that we are able to make of its properties and effects, that the order of treating the subject which we have adopted, will be found, we appre hend, the most convenient. For the present, we may con sider heat, or caloric, to be a principle or power existing in bodies, which gives rise to many of their most important actions, and modifies their effects upon other substances.