ON BODIES SMALLER THAN ATOMS. 329 



meable by the Eontgen rays given out by the tube in which the cor- 

 puscles are produced than when they strike against a target opaque to 

 these rays. I have tested the heating effects produced in permeable 

 and opaque targets, but have never been able to get evidence of any 

 considerable difference between the two cases. The differences actually 

 observed were small compared with the total effect and were some- 

 times in one direction and sometimes in the opposite. The ex- 

 periments, therefore, tell against the view that the whole "of the 

 mass of a corpuscle is due to its electrical charge. The idea that mass 

 in general is electrical in its origin is a fascinating one, although it 

 has not at present been reconciled with the results of experience. 



The smallness of these particles marks them out as likely to 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 realized 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 what the plates were made of, 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. 

 Xow this is exactly what would happen if the atom 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 thus on this view the density 

 and the density alone should fix the resistance offered by the sub- 

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

 cisely Lenard's result, which is thus 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- 



