78 Scientific ProceeMngs, Roi/al Dublin Society. 



our attention), formed each second, depending upon the chain of events 

 leading back to the parent substance, and, of course, upon the amount of 

 this parent substance. Its rate of formation being then fixed, evidently 

 the expectation of life and dependent death-rate, or fraction of the atoms 

 perishing per second, must determine the amount which can be present : just 

 as tlie poptUation of a city must depend upon the number of people born 

 and the number which die every year. When the whole sequence of events is 

 working smoothly, each element steadily forming and being transformed, the 

 amount of each body present is called the equilibrium amount. The amount 

 of radium in equilibrium witli one gramme of uranium is 3'4 ten-millionths of 

 a gramme — a quite unweighable quantity. Boltwood, Strutt, McCoy, and 

 others have verified the fact that this is always the quantity of radium found 

 associated with one gramme of uranium. The existence of these equilibrium 

 amounts of the several elements is the test of the radioactive theory. In the 

 instance cited, it is of considerable practical importance, for it enables a 

 ready and sure estimate to be made of the amount of radium existing in 

 an ore of uranium when the percentage of the latter element has been 

 determined. 



It requires some pliability of mind, at least on the part of the older 

 generation, to recognize that these successive substances are, indeed, distinct 

 and definite chemical elements. But such they are. The case of radium and 

 the substance, emanation, to which it gives rise is particularly interesting. 

 Radium is a metal of the alkaline-earth tj'pe, and nearly akin to the fairly 

 common element barium in its chemical relations. It resists volatilization at 

 liigh temperatures. Radioactively, it is relatively stable, its atoms enduring 

 on the average 2,900 years. Emanation is a gas, only condensed to the liquid 

 form by extreme cold. Chemically, it is highly inert. Radioactively, it is 

 very unstable, the average life being but 65 days. Here the substances differ 

 physically, chemically, and radioactively in the most marked manner: the 

 emanation as widely diverging from the body giving rise to it as the butterfly 

 from the longer-lived chrysalis from which it emerges. 



In the transformation of one element into another those external manifesta- 

 tions arise which occasioned the discovery of the whole matter. Sometimes 

 these take the form of radiant matter; sometimes of the yS and 7 radiation. 

 In a few cases changes can be inferred although no radiation appears. 

 The fact that an atom of a well-recognized element, such as uranium, 

 expels an atom of another definite element, helium, is obviously one of 

 profound significance, the far-reaching nature of which it is impossible to 

 predict. It has certainly completed the downfall of the tottering edifice of 

 the earlier atomistic ideas, and has assisted to furnish those labyrinths which 



