108 McCrietitanp—-The Penetrating Radium Rays. 
Prof. Thomson* has pointed out the significance of this law of absorption in 
the case of charged particles. If the particle be small compared with the molecule 
of the absorbing substance, we must consider collisions as taking place, not with 
the molecule, as a whole, but with the constituents of which it is composed. If, 
then, all substances are built up of the same primordial atoms, the absorption by 
different substances should be proportional to the number of such primordial atoms 
per unit volume, or simply proportional to the density. Thomson also shows 
that the greater the velocity of the particle—the more penetrating the rays—the 
more closely will the absorption density law be obeyed. 
It is not at first sight so clear why such a law of absorption should hold for 
Rontgen rays. If we regard the front of a Rontgen pulse (or a wave of light) 
as uniform and free from structure of any sort, it would seem difficult to explain 
such a law. If, however, we regard a Réntgen pulse as possessing a structure 
(Thomson, ‘ Electricity and Matter,’ p. 63), so that the front of a pulse is not 
uniform, but that, at some places, the forces are great, while larger regions are free 
from force, we can at once get an explanation of our results. 
Such a conception has been theoretically deduced by Thomson, and used to 
explain why, when Rontgen rays pass through a gas, some molecules are ionized, 
while far more are unaffected. 
Applying this idea to the case we are considering, we see that if the regions 
of great force in the pulse are sufficiently small, we must consider their collision 
with the constituents of the molecule rather than with the molecule as a whole. 
If, in addition, the pulse is sufficiently thin—that is, sufficiently penetrating—we 
should deduce the same law of absorption for the Rontgen rays as for charged 
particles travelling with great velocity. 
That a similar result does not hold for ordinary light waves is because this last 
condition is not satisfied. 
NorE ADDED DURING PUBLICATION. 
Two papers have recently appeared by Paschen (Annalen der Physik, Nos. 6 and 7, 1904), in 
which the author claims to show that the y rays carry a negative charge. In the first paper 
the radium is surrounded with lead 1:9 cms. thick, and carefully insulated in a vacuum, when 
the lead gets a positive charge. It should be pointed out that under the action of Rontgen 
rays (according to Curie and Sagnac, Dorn and Thomson) a metal gives out negatively-charged 
corpuscles, thus itself getting a positive charge. The effect observed by Paschen is thus quite 
in agreement with the y rays being of the nature of Rontgen rays. In the second paper the 
author claims to show the existence of negatively-charged radium rays, which are only slightly 
deflectable in a magnetic field, but this paper gives no direct proof of the y rays carrying a 
negative charge. 
* ¢¢ Conduction of Electricity through Gases,” p. 312. 
