580 Professor J. J. Thomson [April 19, 



afford a very valuable means for investigating the details of molecular 

 structure — a structure so fine that even waves of light are on far too 

 large a scale to be suitable for its investigation, as a single wave- 

 length extends over a large number of molecules. This anticipation 

 has been fully realised by Lenard's experiments on the obstruction 

 offered to the passage of these corpuscles through different substances. 

 Lenard found that this obstruction depended only upon the density of 

 the substance, and not upon its chemical composition or physical state. 

 He found that, if he took plates of different substances of equal areas 

 and of such thicknesses that the masses of all the plates were the same, 

 then, no matter of what the plates were made, whether of insulators 

 or conductors, whether of gases, liquids or solids, the resistance they 

 offered to the passage of the corpuscles through them was the same. 

 Now this is exactly what would happen if the atoms of the chemical 

 elements were aggregations of a large number of equal particles of 

 equal mass : the mass of an atom being proportional to the number 

 of these particles contained in it, and the atom being a collection of 

 such particles through the interstices between which the corpuscle 

 might find its way. Thus, a collision between a corpuscle and an atom 

 would not be so much a collision between the corpuscle and the atom 

 as a whole, as between a corpuscle and the individual particles of 

 which the atom consists ; and the number of collisions the corpuscle 

 would make, and therefore the resistance it would experience, would 

 be the same if the number of particles in unit volume were the same, 

 whatever the nature of the atoms might be into which these particles 

 are aggregated. The number of particles in unit volume is, however, 

 fixed by the density of the substance, and on this view the density 

 (and the density alonej should fix the resistance offered by the sub- 

 stance to the motion of a corpuscle through it ; this, however, is 

 precisely Lenard's result, which is a strong confirmation of the view 

 that the atoms of the elementary substances are made up of simpler 

 parts, all of which are alike. This and similar views of the constitu- 

 tion of matter have often been advocated ; thus in one form of it, 

 known as Prout's hypothesis, all the elements were supposed to be 

 compounds of hydrogen. We know, however, that the mass of the 

 primordial atom must be much less than that of hydrogen. Sir Nor- 

 man Lockyer has advocated the composite view of the nature of the 

 elements on spectroscopic grounds, but the view has never been more 

 boldly stated than it was long ago by Newton, who says : 



" The smallest particles of matter may cohere by the strongest 

 attraction and compose bigger particles of weaker virtue, and many of 

 these may cohere and compose bigger particles whose virtue is still 

 weaker, and so on for divers successions, until the progression ends in 

 the biggest particles on which the operations in chemistry and the 

 colours of natural bodies depend, and which by adhering compose 

 bodies of a sensible magnitude." 



The reasoning we used to prove that the resistance to the motion 

 of the corpuscle depends only upon the density is only valid when 



