HEAT. The laws, according to which the temperature of bodies is subject to increase or diminution, have been dis cussed in the articles CALORIC, CAPACITY, COLD, COMBUSTION, and CHEMISTRY. In the first of these articles, caloric was con sidered as a substance capable of passing from body to body, and subsisting in them in different states: Thi§ is the general doctrine of chemical philosophers : many of these, however, as well as others, in cline to the hypothesis, that heat may consist in an undulatory or other intes tine motion, either in the parts of bodies, or in some subtle fluid, or ETHER, which see. Among these, we may reckon Sir Isaac Newton, Mr. Cavendish, Dr. Young, and Count Bamford.
" If heat," says Dr. Young, " when at tached to any substance, be supposed to consist in minute vibrations, and, when propagated from one body to another, to depend on the undulations of a medium highly elastic, its effects must 'strongly resemble of sound, since every sounding body is in a state of vibration ; and the air, or any other medium, which transmits sound, conveys its undulation to distant parts, by means of its elasticity : and we shall find, that the principal phe nomena of heat may actually be illustrat ed-by a comparison with those of sound. The excitation of heat and sound are not only similar, but often identical ; as in the operations of friction and percussion ; they are both communicated sometimes by contact, and sometimes by radiation ; for, besides the common radiation of sound through the air, its effects are com municated by contact, when the end of a tuning•fork is placedon a table, or on the sounding-board of an instrument, which receives from the fork an impression that is afterwards propagated as a distinct sound. And the effect of radiant heat, in raising the temperature of a body upon which it falls, resembles the sympathetic agitation of a string, when the sound of another string, which is in unison with it, is transmitted to it through the air. The water, which is dashed about by the vi brating extremities of a tuning-fork dip ped into it, may represent the manner in which the particles at the surface of a liquid are thrown out of the reach of the force of cohesion, and converted into va pour ; and the extrication of heat, in con sequence of condensation, may be com pared with the increase of sound pro duced by lightly touching a chord which is slowly vibrating, or revolving in such a manner as to emit little or no audible sound ; while the diminution of heat, by expansion, and the increase of the capa city of a substance for heat, may be attri buted to the greater space afforded to each particle, allowing it to be equally agitated with a less perceptible effect on the neighbouring particles. In some cases, indeed, heat and sound not only resemble each other in their operations, but produce precisely the same effects ; thus, an artificial magnet, the force of which is quickly destroyed by heat, is af fected more slowly in a similar manner, when made to ring for a considerable time ; and an electrical jar may be dis charged, either by heating it, or by caus ing it to sound by the friction of the fin.
ger." See the articles first mentioned.
'Isar, animal. The temperature which animals, and even vegetables, maintain, during life, above that of surrounding ob jects, is a very striking phenomenon. By general analogies, it has frequently been referred to the process of combustion ; and, from facts more distinctly pointed, the doctrine, that it depends upon the absorption of oxygen, has been advanced by modern chemists. But it is to Dr. Crawford we are indebted for a direct se ries of experiments, by which the nature of the process is directly made out. It would carry us too far into physiological disquisition, if we were to proceed to en quire respecting the nature of the parts, and the functions of organized beings.— The blood which circul.tes through the lungs absorbs oxygen in the act of respi ration, by means of which a portion of the carbon which it contains is acidified, and carried off in the elastic state. After this, and, perhaps, other changes, the fluid passes through the arteries to the ex treme vessels, depositing, in some man ner, the elementary parts or principles of animal matter during the act of nutrition, in which state of still further change the blood returns by the veins, and again passes through the course of circulation. From his experiments on the capacities of arterial and venous blood, Dr. Craw ford found the capacity of the former for heat to be 1.030, and that of the latter only 0.892, whence he concludes, that though heat must be given out in conse quence of the diminished capacity of the combined oxygen absorbed by respira tion, yet the increased capacity of the ar terial blood will prevent its becoming sen sible immediately in the lungs ; instead of which, it will be given out at the small er ramifications, where the blood be comes changed in its nature, and in its capacity for heat, by its conversion to the venous state. It has also been establish ed, by the experiments of the same philo sopher, that the process of absorption of oxygen is less in a higher than in a low temperature ; the difference between the arterial and venous blood being at the same time less, and consequently the aug mentation of temperature in the animal less considerable. This law ofthe animal economy. assisted by the increased mists ration, and by the slow conducting power of an animal body, and, perhaps, by the permanency of the enlarged capacity, seems sufficient to account for the power which animals possess of maintaining their natural temperature, without any-remark able change in an atmosphere greatly heated, as was shewn in the experiments of Fordyce and Blagden. (See Philos. Trans. 1775.) It must be confessed, how ever, that some farther investigations seem wanting on this subject.
Though the lungs appear to be the great organ of oxygenation in the larger animals, it is well ascertained, that a pro cess of nearly the same nature is carried on at the skin ; and in many of the smaller or less perfect animals there appears to be no other means for effecting this ab sorption.