ll 
Penetrating Radium Rays. Te 
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 Réntgen rays. If we regard the front of a 
Réntgen pulse (or a light-wave) as uniform and free from 
structure of any sort, it would seem difficult to explain such 
alaw. If, however, we regard a Rontgen 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 Réntgen rays pass 
through a gas some molecules are ionized while far more 
are unaffected. 
Applying the 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 Roéntgen pulse as for a charged particle travelling with 
great velocity. Thata similar result does not hold for ordinary 
hght-waves is because this last condition is not satisfied. 
Note 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 Réntgen 
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 guite in agreement with they 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. 
