148 The Hon. Robert John Strutt [March 27^ 



it seems common enough. Now I wish to speak for a httle about 

 this association of uranium and radium in pitch-blende. Is it 

 accidental, or has it some special significance ? I hope to be able to 

 convince you that it has. 



In the early days of radium, it was common to hear the difficulty 

 emphasised that while there was no reason for doubting that the 

 radium which was found in the earth had been there as long as other 

 metals, a substance that was continually giving out energy in this 

 way was obviously defying the greatest physical generalisation of the 

 nineteenth century — the law of the conservation of energy. We 

 cannot, however, afford to sacrifice this law so easily, and a ready 

 mode of escape offers itself, if we suppose that a continual waste of 

 radium is occurring. In that case, it becomes necessary to suppose 

 also that the supply is in some way replenished, for otherwise all the 

 radium would have wasted long ago. From what material are the 

 fresh supplies of radium derived ? They must be derived from some 

 other substance contained in the mineral where the radium is found, 

 and there is now reason to feel sure that uranium is the substance in 

 question. 



We have convincing proof of this, in the fact that the amount of 

 radium found in the mineral is always in direct proportion to the 

 quantity of uranium which it contains. I should, perhaps, say, to 

 avoid misconception, that there is good reason for believing that 

 several transitional stages exist through which uranium passes on its 

 road to become radium. It is not necessary, however, to take into 

 account the existence of these intermediate products in order to form 

 a clear idea of the process by which the supply of radium is kept up. 

 Uranium changes spontaneously though very slowly into radium, and 

 the amount of radium produced per annum, for example, will be pro- 

 portionate to the amount of uranium present. On the other hand, a 

 certain fraction of the total amount of radium present decays per 

 annum, and the balance of this account of profit and loss will 

 represent the amount of radium found in the mineral at any time 

 that we examine it. There will be no difficulty in seeing that on this 

 theory the amount of the radium in the mineral should be pro- 

 portionate to the amount of uranium ; and experiment fully confirms 

 the theory by showing that such is in fact the case. We have here a 

 clear and distinct case of the transmutation of metals, so long 

 unsuccessfully searched for. 



Let us now come back to the pitch-blende. 



What was the source of metalliferous ores found in mineral veins 

 is a very much vexed question, and no solution of it which has yet 

 been proposed can be said to be altogether free from difficulty. One 

 of the most plausible theories, however, supposes that the metals 

 have been derived from the rocks by which the veins are traversed. 

 We are not here concerned with metalliferous ores in general, but 

 only with those which carry radio-active material. In deciding 



