128 RADIOACTIVITY. 



activity of a certain ray which it emits, repeating the experiment 

 in from one to two hours. I then estimate the decay of activity dur- 

 ing this time and thus decide whether I am dealing with a new or a 

 known substance. We are thus in a position, as has been said, to 

 distinguish disintegration products of uranium down to radium G, 

 and to determine their characteristic radiations and half-value times. 



It may be said that in experimenting with radium G the substance 

 disappears in the very hands of the investigator, but that is not sur- 

 prising if we keep in mind the fact that the quantity of material 

 treated in these experiments is so infinitesimal that its presence can 

 be proved only by its radioactivity. Whether it changes into a less 

 active or an entirely inactive substance, it apparently disappears; 

 its mass is too inconsiderable to be weighed, too minute to be detected 

 by the microscope; there is no instrument by which its presence 

 can be proved. Progress is possible in this direction only when we 

 have a larger supply of radium. More radium is the emphatic 

 demand of the medical man, the chemist, and the physicist. So far, 

 therefore, we have been able to decide only indirectly into what 

 radium F is transformed, but, through experiments which need not 

 now be described, the conclusion has been reached that radium F must 

 become lead. 



It is an interesting fact that a large number of minerals which 

 contain uranium always contain helium and radium A-F, and also 

 lead in appreciable quantities. That surely favors the preceding 

 supposition. The task is now to show that such a metal can have 

 originated only from the mother substance — uranium — a portion of 

 which in the course of centuries has passed through the transmuta- 

 tions named; so that now one such specimen contains not only 

 uranium, grandmother, mother, and chrld, but nine generations fol- 

 lowing closely on each other. We could thus trace the genealogy of 

 lead back to uranium w^ithout a break. 



The next question is : Are the less radioactive substances to be con- 

 sidered as evidence of an earlier period of the earth's development; 

 have such substances now reached a fixed, a final condition, and can 

 there at present be shown a single example of the development and 

 transmutation process, such as the other elements have passed 

 through? Or, on the contrary, are all elements radioactive and still 

 in a state of progressive development? We must not at once say 

 " no " to these questions, for the reason that we have not yet observed 

 radioactive qualities in these elements. We need only remember 

 that it is easy to prove magnetic qualities in steel and iron, while in 

 aluminum the apparent magnetic energy is a hundred million times 

 less, and if we had only such substances as aluminum, copper, and 

 zinc at our disposal we might not to-day understand the phenomena 

 of electricity. After an opportunity to study these phenomena with 



