HEAT. 



doctrine of chemical philosophers : many 

 of tiiese, 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 Rumford. 



" 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 those 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 : 



VOL. VI. 



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. 



HKAT, 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 circulates 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 rapacities 

 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 of the animal 

 economy, assisted by the increased evapo- 



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