48 PHYSICAL SCIENCE 



ing could be observed with any of the gases 

 examined. 



The apparatus just described is clearly not 

 adapted to detect small thermal changes, and it 

 was not till about the year 1850, when Thomson 

 and Joule devised a continuous method, that 

 satisfactory results were obtained. Instead of 

 preventing external work by allowing the gas to 

 expand into a vacuum, these physicists performed 

 the external work needed to expand the gas 

 against the pressure of the atmosphere by means 

 of an air-pump driven by an engine. By this 

 method a continuous current of gas was forced 

 through a porous plug of compressed wool or 

 silk, fixed in a wooden tube. Here the engine 

 does the external work, and consequently none 

 of that work draws on the heat energy of the 

 gas itself. 



All the external work is done by the engine, 

 but, as we have seen, another source of energy- 

 change exists. When a gas expands, whether 

 or not it performs external work, the various 

 parts of it become separated further from each 

 other, since, on the whole, the gas occupies after 

 expansion a larger volume than before. If, then, 

 there is any attraction between the parts of the 

 gas, work must be done in separating them ; 

 in terms of the molecular theory, work is done 

 against the inter-molecular forces. For the 

 performance of this internal work, energy must 

 be drawn from the heat-supply of the gas, which 

 will therefore cool, and the amount of cooling, if 

 access of heat from outside be prevented, measures 

 the intensity of the inter-molecular forces. On 

 the other hand, if the inter-molecular forces be 



