298 POPULAR SCIENCE MONTHLY. 



bombardment from a radio-active substance continues without inter- 

 mission and apparently without sign of diminution or cessation. 

 There is every reason to believe that a minute scrap of radium, 

 scarcely perceptible to the eye, may go on emitting these energetic 

 projectiles for hundreds of years. 



11. At first sight the fact that it is merely atoms of matter which 

 are being flung off by most radio-active substances, and that ethereal 

 and other effects are subsidiary to this emission of substance, seems 

 to lessen the interest attaching to the phenomenon, reducing it to 

 something of merely chemical importance, and suggesting a resem- 

 blance to scent or other volatilization from solid bodies. But Pro- 

 fessor Eutherford, with great skill, succeeded in determining approxi- 

 mately the atomic weight of the utterly imperceptible amount of 

 substance thrown off, as well as its speed, and found that it was not 

 by any means the radio-active substance itself which was evaporating, 

 but something quite different. 



Plainly, if an elementary form of matter is found to be throwing 

 off another substance, it becomes imperative to inquire what that sub- 

 stance is, and what it is that is left behind. Now the atomic weight 

 of radium, or of thorium or uranium, or of any known strongly radio- 

 active substance, is very high, in each case over two hundred times the 

 atomic weight of hydrogen, whereas the atomic weight of the substance 

 flung off appears to be more nearly of the order one or two; in other 

 words, the substance thrown off is more likely to be either hydrogen 

 or helium than it is likely to be radium. It is just possible that the 

 inert chemical elements are by-products of radio-activity. 



Now clearly here is a fact, if fact it be, of prodigious importance. 

 Undoubtedly the measurements require confirmation, but for myself 

 I see no reason to doubt them, at least as regards their order of magni- 

 tude. The atomic weight of radium being say 235, and that of the 

 projected portion being say 2, the residue must represent by its atomic 

 weight the difference between the heavy atom of the original substance 

 and that of the light atom or atoms which have been flung away : unless 

 indeed it be assumed, as it will almost certainly be assumed by some 

 skeptical chemists, those who derided argon and other chemical dis- 

 coveries when made in a physical manner, that the substance flung 

 away is some foreign ingredient or impurity — a hypothesis, I venture 

 to say, already strongly against the weight of available evidence. 



The substance left behind in the pores of the radio-active substance 

 has been examined even more completely than the projected portion: 

 it is volatile, it slowly diffuses away, and it behaves like a gas. It can 

 be stored in gas-holders when mixed with air, for in amount it is quite 

 imperceptible to all ordinary tests; and yet it can be passed through 

 pipes and otherwise dealt with. It condenses not far above the tem- 



