166 CATHODE RAYS. 



We see from Lenard's table that a cathode ray can travel through 

 air at atmospheric pressure a distance of about half a centimeter 

 before the brightness of the phosphorescence falls to about one-half of 

 its origiual value. ISow the mean free path of the molecule of air at 

 this pressure is about 10^^ centimeters, and if a molecule of air were pro- 

 jected it would lose half its momentum in a space comparable with the 

 mean free path. Even if we suppose that it is not the same molecule 

 that is carried, the eifect of the obliquity of the collisions would reduce 

 the momentum to one-half in a short multiple of that path. 



Thus, from Lenard's experiments on the absorption of the rays out- 

 side the tube, it follows, on the hypothesis that the cathode rays are 

 charged particles, moving with high velocities, that the size of the 

 carriers must be small compared with the dimensions of ordinary 

 atoms or molecules. The assumption of a state of matter more finely 

 subdivided than the atom of an element is a somewhat startling one; 

 but an hypothesis that would involve somewhat similar consequences, 

 viz, that the so-called elements are compounds of some primordial 

 element, has been put forward from time to time by various chemists. 

 Thus, Prout believed that the atoms of all the elements were built up 

 of atoms of hydrogen, and Mr. Norman Lockyer has advanced weighty 

 arguments, founded on spectroscopic consideration, in favor of the 

 composite nature of the elements. 



Let us trace the consequence of supposing that the atoms of the 

 elements are aggregations of vej^y small particles, all similar to each 

 other. We shall call such particles corpuscles, so that the atoms of 

 the ordinary elements are made up of corpuscles and holes, the holes 

 being predominant. Let us suppose that at the cathode some of the 

 molecules of the gas get split up into these corpuscles, and that these, 

 charged with negative electricity and moving at a high velocity, form 

 the cathode rays. The distance these rays would travel before losing 

 a given fraction of their momentum would be proportional to the 

 mean free path of the corpuscles. Now, the things these corpuscles 

 strike against are other corpuscjes, and not against the molecules as 

 a whole; they are supposed to be able to thread their way between 

 the interstices in the molecule. Thus the mean free path would be 

 proportional to the number of these corpuscles; and, therefore, since 

 each corpuscle has the same mass to the mass of unit volume — that 

 is, to the density of the substance, whatever be its chemical nature 

 or physical state, the mean free j)ath, and therefore the coefficient 

 of absorption, would depend only on the density. This is precisely 

 Lenard's result. 



We see, too, on this hypothesis, why the magnetic deflection is the 

 same inside the tube, whatever be the nature of the gas, for the carriers 

 of the charge are the corpuscles, and these are the same whatever gas 

 be used. All the carriers may not be reduced to their lowest dimen- 

 sions; some may be aggregates of two or more cojpuscles; these would 



