JoLY — Radiant Matter. 79 



science recognizes as existing within space-conditions approximating to those 

 of a mathematical point. 



It is worth noticing liow the discovery of the chemical nature of the a ray 

 harmonizes with sueli atomic weiglits of tliese new elements as liave been so far 

 determined. The atomic weight of uranium is 238'5. There is a loss of three 

 a rays before radium is reached. As the atomic weight of helium is 4, this 

 involves the loss of 12 units of atomic weight. Tlie atomic weight of radium 

 should, therefore, be 226'5. This figure closely approximates to the results of 

 the best determinations. Again, radium loses an a ray and becomes emanation. 

 The atomic weiglit of the latter sliould therefore be 222"5. Recent investiga- 

 tions — more especially those of Gray and Ramsay — give results approximating 

 closelj' to this figure. In the uranium series of changes eiglit a rays are lost 

 altogetlier. The residual body must possess the atomic weight 206'5. Now, this 

 is nearly that of lead; and Boltwood has accordingly suggested that the final 

 bodj' is indeed that element. 



As ages pass over, the parent substance must diminish in amount. Finally, 

 it must all disappear, and all its descendants along with it, save tlie stable 

 body to which it ultimately gives rise. The radiated matter, the lielium, also 

 survives. The disajjpearance of all the uranium will take an immeuse time. 

 Six thousand million yeai-s will see one half the present store of uranium used 

 up ; another sis thousand million years will be required to reduce it to one 

 quarter, and so on. 



When we hear such statements as the foregoing, we are inclined to tliiuk 

 that the radiant matter must be evolved at a very slow rate. The figures 

 regarding the rates of emission of o rays create a very different impression 

 on the mind. Thus, Rutherford, who has succeeded by a most ingenious 

 method in counting the individual a rays projected from radioactive matter, 

 has shown that one gramme of uranium, with all its products of change 

 associated with it — that is, in fact, as we find it in the rocks — emits nearly 

 100,000 helium atoms per second ; more exactly, 9'67 x 10*. One quarter of 

 the number of these departing helium atoms signals the downfall of the 

 uranium atoms — the transmutation of entities which have lasted probably 

 some thousands of millions of years. The new atoms will never again be 

 uranium ; but in the course of a few thousand years, during which they run 

 down the scale of energy, they again find an enduring stability. It is an 

 impressive thought that, notwithstanding this rapid loss of atoms from a 

 single gramme of uranium, so vast an interval as the half-period must elapse 

 before one-half the number of atoms present have been transformed. 



The major part of the heat which radioactive substances continually generate 

 in the rocks, the geological significance of which is so important, is due to the 



