220 THE REALITIES OF MODERN SCIENCE 



This method was first used by Joule, who found no 

 change in temperature. The conclusion reached above, 

 as to the equivalence between external work and 

 energy input in the form of heat, when an expanding 

 gas is maintained at constant temperature, is known 

 as Joule's Law. Later and more precise experiments 

 showed that all gases cool somewhat upon free ex- 

 pansion, except that for hydrogen there is an anomalous 

 warming effect at ordinary temperatures. 



If Joule's Law does not hold we are not at liberty to 

 use the equation PV=RT except as an approximation. 

 It is convenient, nevertheless, in considering gases to 

 discuss the " perfect gases," that is, imaginary gases, 

 for which PV = RT would always be true. Actual 

 gases depart from this perfect gas relation not only 

 because the molecules occupy some space, but also 

 because the compression of a gas is not due solely to 

 the externally applied pressure. The molecules are 

 brought closer together by the tractation which ac- 

 companies the decrease in p.e. of the systems which 

 they form. Except, however, for temperatures near 

 that at which the gas liquefies the perfect gas equation 

 may be used to determine the condition of an actual 

 gas. 



In liquids the molecules are very close together. 

 A molecule in the body of a liquid moves without any 

 effect from those molecular attractions which are due 

 to its potential energy with the other molecules. This 

 is because the other molecules are disposed about it in 

 spherical shells. For any molecule of such a shell 

 there is always diametrically opposite another mole- 

 cule. With these two the particular molecule at the 



