If the expansion between H and G is either increased or diminished by a variation in the source of heat, the other expanded and contracted portions suffer a similar change; so that there must always be an equilibrium among the forces, by which these opposite mechanical states are pro duced.
Let the plate of glass ABDC, Fig. 6. be cut with a dia mond through its centre 0, by a line MN, but not separat ed into two parts, and in this state let it be placed with its edge CHD on a hot iron, as before. It will now exhibit the very same phenomena as those which we have already de scriheo, just as if it had never been touched with the dia mond. When it is in this state of contraction in the mid dle, and of expansion at tne edges, let it be suddenly broken through at the line MON, and it will be found that each of its halves A BNN1, MNDC. Hg. 6. arc thrown into the same state of contraction and expansion as when the plate was unbroken. Between E and f, g and 0, 0 and f', g' and If, the glass will be expanded, while between f and g and f' acid g' it will he contracted, as shown in Fig. 7.
Hence, it is obvious, that the contraction and expansion, or rather the phenomena by which they are indicated, are produced by some fluid, which is decomposed by heat into two fluids possessed of opposite polarity, in the same man ner as the electric fluid of the tourmaline is decomposed by the action of heat, or the magnetic fluid of a piece of iron by the action of a loadstone. All the phenomena, indeed, which are exhibited during the passage of heat through a plate of glass, are precisely the same as the phenomena which take place in the communication of magnetical and electrical polarity ; and if we admit the existence of two fluids as the agents by which the phenomena of electricity and magnetism are produced, we are compelled to make the same admission in the case of the propagation of heat along glass. It is not unlikely that the phenomena may be produced by the mutual action of free and latent heat. In our article OPTICS we shall have occasion to resume this subject at great length, and to give drawings and descrip tions of the various optical phenomena which are produced by the singular state, into which glass and other bodies are thrown during the propagation of heat through their sub stance. Those who wish for full information on this sub ject may consult the•Philosolzhical Transactions for 1816, part i. p. 46-114, and 156-179.
The Effects of Heat.
WE now arrive at the second of the great divisions into which we proposed to arrange our materials, the effects of heat. The principal of these are four. 1. Temperature,
or the production of the sensation of heat ; 2. Expansion, or the simple augmentation of the bulk of a body, without altering its condition ; 3. A change in the physical state of a body, by which a solid is converted into a fluid, and a fluid into a gas ; and, 4. Ignition, or the operation of heat, by which a body is rendered luminous.
I. Temperature is often used rather as a generic term, applicable to all the phenomena that bear a relation to heat, than as a specific term, proper to designate any one of them in particular. It is, however,•not unfrequcntly used to ex press the power of exciting the sensation of heat, or the ef fect which it produces on the animal body ; and there seems to be some propriety. in establishing this restriction, be cause the word heat, which might otherwise be employed, as we have already remarked, is liable to the objection of being sometimes used to express the cause, and at other times the effect. The temperature of a body varies accord ing to the quantity of heat which it contains, and may be said to be in the direct ratio of this quantity, although the indica tion of temperature, which we derive from the sensations, is much too vague to enable us to lay down an exact scale of admeasurement. This proportion of effect, however, only applies to the same body, or to those of a similar kind ; for it has been found that the same quantity of heat affects the temperature of different bodies in very different de grees. These different effects depend partly upon the ca pacity of bodies for heat, and partly upon their conducting power ; a point to which we have already referred in a for mer part of the article. . The sensation of temperature is also very much influenced by the state of the nervous sys tem, independent of the quantity of heat in the substance exciting it. It is well known that, when the hand is cold, a body that is applied to it will appear hot, when the same body would excite the sensation of cold, had the hand been previously exposed to a higher temperature. This sub ject belongs more to physiology than to natural philosophy in general ; we shall therefore only remark, that we seem always to compare our present feeling's with our former ones, and to refer our sensations, not to any invariable stan dard, but to what we have felt just previous to the present impression.