president's address SECTION A. 25 



The refinement of the experiment will be appreciated when it is 

 recalled that the rate of production of the helium is only 4x 10~*cc. 

 per gm. of thorianite per year. He found as already mentioned 280 

 million years for one specimen, 250 millions for another. 



To deduce a minimum age for the mineral it must be assumed 

 that :— 



(1) There was no original store of helium in the mineral when 



it was formed. 



(2) The mineral has not gained helium at any time except as 



it does now. 



(3) That the present rate of accumulation of helium is the 



same as in the remote past, when possibly high pressures 

 and temperatures pertained. 

 We will consider the observational basis for these assumptions : 

 (1 and 2) If the helmm was originally present in the mineral 

 when it was formed, or added later, then we would expect to 

 find other minerals, in which helium is not now accumulating, 

 containing helium, but no such minerals are known^ Helmm is 

 only found in appreciable quantities asssociated with thorium and 

 uranium. The mechanism- of how it is continuously and un- ' 

 changingly produced from these elements is known in great detail. 

 (3) That radio-active changes are independent of temperature 

 and pressure has been repeatedly tested and confirmed. 



Relative Motion of Earth and Ether. 



All optical phenomena observed till the classic experiment of 

 Michelson and Morley agreed with the view that the earth moved 

 through an " absolutely " stationary ether^. That experiment, and 

 others, designed to exhibit this relative motion of the earth and 

 ether failed even to show a second order effect. Fitzgerald and 

 Lorentz, by postulating that a body when set in motion contracts 

 in length, were able to reconcile the absence of positive effects, with 

 the hypothesis that the ether is stationary and undisturbed by even 

 the earth's momentum. But the contraction hypothesis of these phy- 

 sicists, though it most ingeniously accounted for the null effects, where 

 positive ones were expected, appeared neither capable of proof 

 or of disproof, and so to many was unconvincing. Lorentz showed 

 that the shrinkage hypothesis required that the mass of moving 

 electric charges [ft and cathode rays) should increase in a certain 

 definite manner with their speed of translation. This has since 

 been exactly confirmed. And so the hypothesis becomes at once 

 one of the most remarkable and historically interesting in physics. 

 Einstein approaching the experimental evidence from a different 

 standpoint, postulates that it is impossible to detect uniform 

 motion of the observer and his instruments relative to the ether. 



1. The mineral beryl, it should be stated, has been found bv Strutt to have an excep- 

 tional high ratioof helium to uranium, but both Strutt and Boltwood suggest explanation'; 

 Prcc. Roy. Soc, 84, A 569, p. 194., July, 1910. 



2. See for example Rutherford's Nobel Lecture, 1908. 



3. See Bumstead, Amcr. Journ. Set., Nov., 1908. 



