Mr. W. J. M. Rankine on the Mechanical Action of Heat. 9 

 periraents on the expansion of atmospheric air, carbonic acid and 

 hydrogen*. . , 



In a perfect gas, the abpve expression is reduced to 



r=^^H$-0 '™-' 



Let n, as before, denote the number of atoms of a substance 

 which, in the state of perfect gas, occupy unity of volume under 

 unity of pressure at the temperature of melting ice, so that niW 

 is its specific gravity in that state : then 



f=a'"'<$+0- • • • (™-' 



The factor by which ^ is here multiplied fulfills the con- 



2 



ditionofbeing a function of y, and of constants which are the 



same for all substances, and is therefore fitted for a measure of 

 temperature. It obviously varies proportionally to the pressure 

 of a perfect gas of a given density, or its volume under a given 

 pressure. „ 



Let T, therefore, denote temperature, as measured trom an 

 imaginary zero, C degrees of the scale adopted, below the tem- 

 perature of melting ice, at which 



then for all substances 



and in perfect gases 



^ CnM 



T may be termed absolute temperature, and the point from 

 which it is measured, the absolute zero of temperature. This, as 

 I have observed, is an imaginaiy point, being lower than the 

 absolute zero of heat by the quantity C«^6, which is the same 

 for all substances. 



The value of C, or the absolute temperature of melting ice, as 

 determined from M. Regnault's experiments, is 



274°-6 Centigrade, 

 being the reciprocal of 



0-00364166 per Centigrade degree, 

 the value to which the coefficients of dilatation of gases at the 

 temperature of melting ice approximate as they are rarehcd. 

 • Philosophical Magazine, December ISf)!. 



(IX.) 



