57 6 SCIENCE PROGRESS 



address on the " Atomic Theory " to the British Association 

 in Glasgow in 1901, or in Clerk Maxwell's earlier articles on 

 the same subject. 



Two recent developments have gone to make clearer than 

 ever before the essential difference between the chemist's atom 

 and the physicist's molecule, and also the artificialities that have 

 crept alike into chemical and physical science by the unconscious 

 effort to give to the molecule the generality of application 

 which in fact applies only to the atom. In the first place, 

 Perrin's successful application of Avogadro's law to microscopic 

 particles of solids, such as gamboge, suspended in liquids, 

 particles which contain probably billions of molecules of 

 gamboge, in the ordinary chemical sense of the word, is a 

 striking proof that no conception whatever of ultimateness 

 enters into the molecular theory. The size of the particle is 

 of no significance. The sole exact definition of a molecule in 

 the physical sense is that it is a separate particle capable of 

 free and unhampered, or better un-anchored, movement, in 

 which movement it is subject to free and unceasing interchange 

 of kinetic energy by collision with other similar freely moving 

 particles. Under such conditions of free motion and mutual 

 collision with interchange of kinetic energy the average 

 amount of kinetic energy of translation which each molecule 

 secures is the same for all molecules independently of their 

 mass, and is simply proportional to the absolute temperature. 

 The ideal gas laws, pv — constant, or pv = RT, on which the 

 kinetic theory of gases has been founded, are expressions of 

 the foregoing generalisation, the product /^ being a measure of 

 energy, which is proportional to the kinetic energy of translation 

 of the individual particles or molecules of the gas, and quite 

 irrespective of the nature of the gas or whether its molecules 

 are light or heavy. 



The molecule, unlike the atom, is thus not distinguished by 

 its mass, but by the fact that, independent of its mass, it pos- 

 sesses a definite kinetic energy of translation or sensible heat 

 energy at any definite temperature. This test, as Avogadro 

 first accomplished, enables us to determine the mass and 

 therefore the number of atoms in the molecule, to distinguish 

 the lighter from the heavier and to select those which 

 are ultimate particles in the chemical sense of being the 

 smallest particle of the compound which can exist and which 



