284 



and temperature depending on the law of the influence of molecular 

 attraction and repulsion upon the superficial-atomic elasticity. 



This section concludes by contrasting the author's theory with 

 that of Carnot, which has hitherto been followed, either explicitly or 

 virtually, in all calculations respecting the motive power of heat 

 (except in the investigations of Mr Joule, already referred to), and 

 of which a very clear and able account, with copious illustrations, 

 was read before the Royal Society of Edinburgh, in January 1849, 

 by Professor Thomson, Carnot considers heat to be something of a 

 peculiar kind, whether a condition or a substance, the total amount 

 of which, in nature, is incapable of increase or diminution. It is 

 not, therefore, according to his theory, convertible into mechanical 

 power, but is capable, by its transmission through substances under 

 particular circumstances, of causing mechanical power to be deve- 

 loped which did not before exist. According to the author's theory, 

 on the contrary, as well as to every conceivable theory which regards 

 heat as a modification of motion, the production of expansion by 

 heat, and of heat by compression, consist in the transformation of 

 mechanical power from one shape into another. 



The second section relates to real and apparent specific heat, espe- 

 cially in the state of perfect gas. The apparent specific heat of a 

 given substance is defined to be the sum of the real specific heat, 

 and of that heat which is employed in producing those changes of 

 volume and of molecular condition which accompany an elevation of 

 one degree in the temperature of the substance. The same sub- 

 stance may therefore have different apparent specific heats, accord- 

 ing to the manner in which the volume is made to vary with the 

 temperature. The general algebraical expression for apparent spe- 

 cific heat is deduced from the equations of the preceding section. 

 That expression being applied to the case of a perfect gas, or of a 

 gas which may be treated in practice as sensibly perfect, it is shewn 

 that the apparent specific heat of such a gas, at constant volume, is 

 sensibly equal to the real specific heat, and that the apparent specific 

 heat at constant pressure exceeds the specific heat at constant volume 

 in a ratio which is sensibly constant for a given gas. Laplace's 

 method of calculating this ratio from the velocity of sound is referred 

 to, and applied to atmospheric air, oxygen, and hydrogen, using the 

 correct coefficients of dilatation of those gases, as determined by 

 M. llegnault. 



