82 Mr. W. Sutherland on the Law of Attraction 



It will be remembered that Joule first attempted to get 

 evidence of the molecular attractions or repulsions in a gas 

 by allowing air to escape from under pressure in one vessel 

 into vacuum in another, and by measuring the difference of 

 temperature of the gas before and after. Stated as a general 

 method, capable of application to all bodies to measure the 

 alteration of their potential energy with the distance of their 

 molecules, it is this : — Allow the body to pass instantaneously 

 from one state to another without doing external work; the 

 heat developed is the thermal equivalent of the change of 

 potential energy. M. Edlund (Poggendorf, cxxvi.) made a 

 study of some metal wires in this manner, but they were 

 stretched, and of course the stretching had to be kept within 

 the elastic limits. • The application of these experiments was 

 limited to the verification of one or two thermodynamic 

 relations. To obtain anything closer than a first approximation 

 to the law connecting the potential energy and dimensions of 

 bodies, it would be necessary to subject liquids and solids to 

 pressures increasing to the greatest possible extent, and mea- 

 sure their change of potential energy, when released, by the 

 thermal effect. This law once obtained, the deduction of the 

 law of force would be a pure question of mathematical analysis. 



Joule's first method not proving delicate enough for the 

 case of gases, he joined Thomson in the series of experiments 

 referred to above. Their method may briefly be described 

 thus : — 



Compressed gas was allowed to expand through a porous 

 plug into the atmosphere. It was always brought to a 

 constant measured temperature on the high-pressure side 

 of the plug, and its temperature was taken on the low-pressure 

 side. It was found to be cooled. However, part of the cooling 

 effect could be traced to the departure of the gas from Boyle's 

 law in this manner: — A volume v 1 of the compressed gas in 

 expanding through the plug to volume v would have work joV 

 done on it by the gas behind (p / being the high pressure), 

 while it would do work pv on the atmosphere in front. But 

 in all gases except H, at about normal pressure and tem- 

 perature, pv >p'v f . Hence, on the whole, the expanding gas 

 does external work, and must accordingly draw on its supply 

 of heat and get cooled. The thermal equivalent of pv— p'v ! 

 at about 15° was calculated by Thomson and Joule from known 

 data for air and C0 2 , and was found to represent in the one 

 case about a fourth, in the other about a third, of the actual 

 cooling. The rest of the cooling effect is due to a gain of 

 potential energy by the molecules at the expense of their heat ; 

 in other words, the molecules of the expanding gas separate 



