RELATIONS OF HEAT TO FORCE. 457 



Tillet recorded an instance of a young girl who remained in an oven for ten 

 minutes without inconvenience, at a temperature of 324-5 Fahr. (162-5 C.). 

 Blagden, in his noted experiments in a heated room, made in connection with 

 Banks, Solander, Fordyce, and others, found in one series of observations, that 

 a temperature of 211 Fahr. (99-5 0.) could be easily borne; and at another 

 time the heat was raised to 260 Fahr. (126-5 C.). Under these extraordi- 

 nary external conditions, the body is protected from the radiated heat by 

 olothing, the air is perfectly dry, and the animal temperature is kept down 

 by increased evaporation from the surface. 



It is a curious fact that after exposure of the body to an intense, dry heat 

 or to a heated vapor, as in the Turkish or Russian baths, when the general 

 temperature is somewhat raised and the surface is bathed in perspiration, a 

 cold plunge, which checks the action of the skin almost immediately, is not 

 injurious and is decidedly agreeable. This presents a striking contrast to the 

 effects of sudden cold upon a system heated and exhausted by long-continued 

 exertion. In the latter instance, when the perspiration is suddenly checked, 

 serious disorders of nutrition, with inflammation etc., are liable to occur. The 

 explanation of this seems to be the following : When the skin acts to keep 

 down the temperature of the body in simple exposure to external heat, there 

 is no modification in nutrition, and the tendency to an elevation of the ani- 

 mal temperature comes from causes entirely external. It is a practical ob- 

 servation that no ill effects are produced, under these circumstances, by sud- 

 denly changing the external conditions ; but when the animal temperature 

 is raised by a modification of the internal nutritive processes, as in prolonged 

 muscular effort, these changes should not be suddenly arrested ; and a sup- 

 pression of the compensative action of the skin is liable to produce disturb- 

 ances in nutrition, often resulting in inflammations. 



RELATIONS OF HEAT TO FORCE. 



Since the development of the theory of the conservation of forces, which 

 had its origin in an essay published by J. R. Mayer, in 1842, physiologists 

 have applied -the laws of correlation and conservation of forces to operations 

 involving the production of heat and the development and expenditure of 

 force in animals. This theory, if applicable to what wei v e formerly called 

 vital operations, certainly affords, in its definite quantities of heat and force as 

 expressed in heat-units and foot-pounds, a basis for calculating the absolute 

 value of material changes in the body. Without discussing the purely physi- 

 cal questions involved, the laws of correlation and conservation of forces, as 

 they are applicable to human physiology, may be briefly stated as follows : 



Potential energy is something either residing in or imparted to matter, 

 which is capable of being converted directly or indirectly into heat. The 

 .animal body, for example, is a store-house of potential energy. Its tissues 

 may be made to unite with oxygen and heat is produced. Any body may 

 have potential energy imparted to it. If a weight be raised to a certain 

 height, when the force which has accomplished this work is exhausted, the 

 potential energy imparted to the weight causes it to fall, and in this fall, heat 



