PRESENT PROBLEMS 269 



it decayed, it was reproduced from its parent thorium at an equal rate. 

 Here is a case analogous to what was sought for with radium and 

 uranium; but evidently uranium is not the only parent of radium; 

 the operation is not one of parthenogenesis. Similar facts have been 

 elicited for uranium by Crookes. 



The a rays, caused by the disintegration of radium and of its eman- 

 ation, are accompanied by rays of quite a different character; these 

 are the /? rays, identical with electrons, the mass of which has been 

 measured by J. J. Thomson and others. These particles are projected 

 with enormous velocity, and are capable of penetrating glass and metal 

 screens. The power of penetration appears to be proportional to the 

 amount of matter in the screen, estimated by its density. These elec- 

 trons are not matter; but, as I shall relate, they are capable of causing 

 profound changes in matter. 



For the past year, a solution of radium bromide has been kept in 

 three glass bulbs, each connected to a Topler pump by means of capil- 

 lary tubing. To insure these bulbs against accident, each was sur- 

 rounded by a small beaker; it happened that one of these beakers con- 

 sisted mainly of potash glass; the other two were of soda glass. The 

 potash-glass beaker became brown, while the two soda-glass beakers 

 became purple. I think there is every probability that the colors are 

 due to liberation of the metals potassium and sodium in the glass. 

 They are contained in that very viscous liquid, glass, in the colorless 

 ionic state; but these ions are discharged by the /? rays, or negative 

 electrons, and each metal imparts its own peculiar color to the glass, 

 as has been shown by Maxwell Garnett. This phenomenon, however 

 interesting, is not the one to which I desire to draw special attention. 

 It must be remembered that the beakers have been exposed only to 

 /? rays; a rays have never been in contact with them; they have 

 never been bombarded by what is usually called matter, except by the 

 molecules of the surrounding air. Now these colored beakers are 

 radioactive, and the radioactive film dissolves in water. After careful 

 washing, the glass was no longer radioactive. The solution contains 

 an emanation, for on bubbling air through it, and cooling the issuing 

 air with liquid air, part of the radioactive matter was retained in the 

 cooled tube. This substance can be carried into an electroscope by 

 a current of air, after the liquid air has been withdrawn, and as long 

 as the air-current passes, the electroscope is discharged; the period of 

 decay of this emanation, however, is very rapid, and on ceasing the 

 current of air, the leaves of the electroscope cease to be discharged. 

 In having such a short period of existence, this emanation resembles 

 the one from actinium. 



Owing to the recess, only a commencement has been made with the 

 investigation of the residue left on evaporation of the aqueous solu- 

 tion. On evaporation, the residue is strongly active. Some mercur- 



