Law of Molecular Force. 183 



more likely to be due to a slight misestimation on Regnault's 

 part of the small differences of his ratios from 1 than to 

 capillary action in Amagat's tube, which, at the temperature 

 and pressure in question, must be slight ; but the point is one 

 not unworthy of being inquired into, whether the slight discord 

 between Regnault's and Amagat's experiments is due to 

 capillary action. 



To determine from the equation the cooling-effects expe- 

 rienced by air in passing through porous plugs, we can write 

 it in the form 



= aAlog — -i-acAl ) +l( ) -h^'a — Pi v v 



Here, as before, ~K p 8 is the quantity of energy which has to be 

 imparted to the gas on the low-pressure side to bring, not the 

 kinetic energy, but the temperature of the gas to its original 

 value on the high-pressure side. If we subtract p2^2~P) v i 

 from this, and neglect the term involving c, on account of its 



smallness, we have aAlog — + l(— 1 as the amount of 



energy imparted to compensate not only for conversion of 



the amount of kinetic energy l( J into potential energy, 



1 . V2 v 



but also to compensate for the cooling a A log — which would 



occur even in a gas for which 1 = 0, or in the ideal perfect 



s as - 



Many writers have ignored this result of the difference A 

 which Thomson and Joule demonstrated to exist between the 

 temperature of melting ice, as measured on the absolute 

 thermodynamic and the air-thermometers, and they have 

 asserted that, for a perfect gas whose equation is pv = aT the 

 cooling-effect obtainable must be zero ; whereas, while the 

 equation for air at temperatures near 100° C. may be written 

 pv = aT, Thomson and Joule were able to demonstrate a quite 

 measurable cooling-effect at that temperature. Before the 

 cooling-effect for a body can be zero, its equation must be 



