48 SECTIONAL ADDRESSES. 



spontaneously go out of existence on the other. Then we have to explain 

 why these heavy atoms are not found on the earth, which, it is generally 

 agreed, originally formed part of the same mass as the sun. Jeans 

 mentions this difficulty, and gives reasons for thinking that the heavy 

 elements would sink to the interior of the mass, so that the earth, formed 

 from the exterior part of it, would not contain them. That a vera causa 

 is here appealed to cannot be doubted ; but there seem to be some 

 difficulties in assuming that it operates with enough precision to secure the 

 desired result. 



The list of known elements ends with uranium, and we must notice 

 that the occupants of the 92 places up to and including uranium in the 

 list, nearly all answer to their proper numbers when the roll is called. 

 The only exceptions are 85 and 87. And he would be a rash philosopher 

 who attached much importance to these vacant places, which may be 

 filled up any day. Roughly speaking, we may say that the elements up 

 to uranium are all present, and the higher members assumed to exist in 

 the stars are all absent. It is putting a heavy burden on the mechanism 

 of gravitational separation to expect it to achieve this result. The 

 inventors of ore-dressing machinery would, I should imagine, despair of 

 accomplishing anything like it. 



Nature works on a vast scale and with plenty of time at her disposal, 

 and it may well be urged that we must be careful of measuring her possible 

 achievements by our own. We may ask, however, whether the more 

 direct indications available suggest that she has in fact made this separa- 

 tion. If there is this cut between the atomic numbers 92 and 93, we 

 should expect most of 92 to have gone into limbo in order to ensure the 

 whole of 93 having done so. Yet 92 (uranium) is a relatively abundant 

 element compared with most, being in fact No. 25 on the list of abundance 

 in igneous rocks, according to the estimate of Clark and Washington. 

 Again, we happen to be in a position to say that on the earth at least, 

 uranium, so far from having sunk to the centre, is concentrated near the 

 surface. This is inferred from the known outflow of heat from the earth, 

 which is difficult to reconcile with the observed amount of radioactive 

 matter near the surface, and impossible to reconcile with the existence of 

 a comparable amount in the interior. 



Assuming that uranium exists on the sun as on the earth, then, as 

 first pointed out by Lindemann, there are strong grounds for thinking 

 that it must be in course of formation there, for the life of uranium is too 

 short in comparison with the probable age of the sun to allow us to suppose 

 otherwise. Those who remember the early development of radioactivity 

 will recall that a parallel argument was successfully used by Rutherford 

 to prove that radium must have originated on the earth before the fact 

 was directly proved that it is being generated here. Radium (it was 

 later shown) is generated from a parent body of higher atomic weight, 

 namely uranium. Jeans would regard the origin of uranium itself as 

 analogous, and if this analogy is accepted it would require the presence 

 of an element of still higher atomic weight, capable of undergoing radio- 

 active disintegration, but, it is to be observed, incapable, ex hypothesi, 

 of dissolving entirely into radiation. 



No doubt these are very deep waters, and we can hardly expect at 



