15 PRESSURE OF GASES 29 



Introducing this absolute temperature, we have for the 

 mean molecular speed the formula 



where H is a constant, the meaning and value of which are 

 easily decided. 1 The molecular speed, therefore, is a mag- 

 nitude which increases proportionally to the square root of 

 the absolute temperature. 



This determination of the absolute zero and of absolute 

 temperature deserves the more notice, as it is the same, or 

 very nearly the same, for all gases ; for, as experiment 

 shows, all gases have very nearly the same coefficient of 

 expansion, 2 and therefore the position of the absolute zero 

 is approximately the same for all gases. 



A real meaning, however, is perhaps not to be ascribed 

 o the zero 3 so found in the sense of its really denoting a 

 temperature at which all molecular- and heat-motion ceases 

 so that there is no more heat. For it is evident that if 

 there is no more heat-motion there is also, according to 

 our theory, no more tendency to expand, and therefore no 

 body can remain in the gaseous state. The formula of 

 Gay-Lussac's law cannot strictly be applied at such low 

 temperatures, since possibly and certainly for such gases as 

 are condensed to liquids at temperatures above 273 C. 

 this law loses its validity at some higher point, and another 

 takes its place. Th&jabaol n te. ~taro perature that has been 

 introduced has therefore more the signification of an auxiliary 

 mathematical function than of a physical reality. 



the freezing-point of water viz. 32 F. and in the case of air is taken to be 

 - x 0-003670 = 0-002039, whereas in the text an initial volume corresponding 



to & = is required on all scales ; so that for air on the F. scale we must take 

 a = 0-002039 -5- (1 - 0-002039 x 32) = 0-002181. TB.] 



1 [In fact, H = 485 -=- V (272-5 x s) = 29'38/ Vs metres per second for the 

 Centigrade scale. TB.] 



2 See 32. 



3 On other determinations of the absolute zero of temperature see Gehler's 

 Physikalisches Worterbuch, x. p. 115. 



