Degrees of Heat in Absolute Measure. 63 



The absolute unit of heat is defined as being the quantity of 

 heat equivalent to the absolute unit of work. If, now, the degree 

 of heat be defined as the increase of temperature which produces 

 an absolute unit of heat in being imparted to the unit of mass 

 of water, the degree of heat is arbitrarily determined, since it 

 depends on the physical nature of the substance chosen, namely 

 water. If instead of a certain quantity of water we select a cer- 

 tain number of atoms of an element, then, in accordance with 

 Dulong and Petit's law, the heating which a given quantity of 

 heat produces in these atoms is independent of the nature of 

 the substance, and it only remains to settle more precisely the 

 number of atoms to be chosen. 



The law in question does not hold quite accurately with re- 

 ference to the solid elements ; yet the deviations are explained 

 by the fact that heat is not only used for heating, but also for 

 performing internal work. On the contrary, the law certainly 

 holds for all those gases in reference to which it can be assumed 

 that none of the heat is consumed in internal work. The loss 

 of heat in external work can be avoided by heating the air under 

 constant volume. 



According to Regnault, we have for constant pressure the spe- 

 cific heats 



Nitrogen. Oxygen. Hydrogen. 



0-24380 0-21751 3*40900 



Hence, to heat under constant pressure 14 mgrms. nitrogen, 

 16 mgrms. oxygen, 1 mgrm. hydrogen, there are required 



3-41320 3-48016 3-40900 



relative units of heat (1 mgrm. water through 1° C). These 

 three numbers, of which especially the first and last are very 

 near each other, show, in accordance with Dulong and Petit' s 

 law, that the same quantity of heat is required to raise the same 

 volume at the same pressure, and therefore also, as we assume, 

 the same number of atoms of the gases in question, through one 

 degree. 



Under constant volume the specific heat of these gases is in the 

 ratio 1*40 : 1 less (1*41 on the old, and 1-3945 from Regnault's 

 determinations of the velocity of sound in air) ; if of the above 

 three numbers we take the mean value of the two which most 

 closely agree with each other (for nitrogen and hydrogen) — that is, 



3-4111, 



we obtain 2*436 thermal units (1 mgrm. of water through 1° C.) 

 as the quantity of heat required for heating under constant 

 volume as many atoms of a permanent gas as are contained in 

 1 mgrm. of hydrogen. 



