SOME ASPECTS OF THE ATOMIC THEORY 577 



if further subdivided will give rise to new substances different 

 in properties from the original. This process of selection, and 

 the fact that of all the molecules containing any one element, 

 some one or more are always found which contain but one 

 atom of that element, represent the invaluable use chemists 

 have made of the molecular theory in their determination of 

 relative atomic weights. 



But the molecular theory applies to any size of particle of 

 any complexity. It is limited only by the condition of free 

 interchange of kinetic energy of translation. The quantum of 

 kinetic energy of translation which each molecule possesses at 

 a definite temperature being fixed, the velocity of translation 

 is inversely proportional to the square root of its mass. This 

 velocity — in the case of hydrogen at ordinary temperature 

 rather more than a mile a second — is in the case of fine micro- 

 scopic particles still great enough to endow them with the 

 most animated Brownian movement, whilst in the case of 

 particles large enough to be perceived by a simple lens, it is 

 still appreciable. Beyond this it becomes inappreciable to 

 measurement. It is this and only this cause which limits the 

 further application of the gas laws on which the molecular 

 theory is based to suspensions of particles large enough in- 

 dividually to be seen by the naked eye. But in imagination 

 it may be extended to a collection of billiard balls floating in 

 a liquid of equal specific gravity, each one of which would be 

 found to be in perpetual Brownian motion on the average 

 with the same amount of kinetic energy as that possessed by 

 a hydrogen molecule at the same temperature, were it not for 

 the fact that the velocity of so great a mass corresponding to 

 this minute quantum of kinetic energy is altogether inappreciable 

 by experiment. 



When we contrast with such a conception of a molecule that 

 usually entertained by the chemist, we bring in at once the con- 

 ception of ultimateness derived from the Atomic Theory. The 

 chemist's molecule is a conception independent of the dynamical 

 law of the equipartition of energy on which the physical 

 molecular theory is based. It stands in the same relation to 

 compound substances as the atom does to elementary sub- 

 stances, and is the ultimate particle of such a compound 

 substance, or the particle than which nothing smaller can exist, 

 exhibiting unchanged the properties of the substance. True, 



