Vol. 65.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxxix 



after the expiration of some unknown period, the investigator 

 should be able, under the conditions which he can command in the 

 laboratory, to liberate the helium and to determine the ratio of its 

 mass to that of the radium, he will then be provided with the data 

 required to determine the time which has elapsed since the first 

 formation of the mineral, presuming of course that he knows the 

 rate at which the helium is generated. The problem is complicated 

 by the fact that in the long run it is not so much the radium, as the 

 uranium from which it is derived, that has to be taken into 

 account : a fact of importance from more than one point of view. 



A preliminary investigation, for which phosphatic nodules 

 furnished most of the material, afforded Mr. Strutt the following 

 results : — 



Years. 



Phosphatic nodules from the Crag 225,000 



Upper Greensand 3,080,000 



Lower Greensand 3,950,000 



Haematite overlying Carboniferous Limestone 141,000,000 



Mr. Strutt regards these periods as the minimum ages, since some 

 ■of the helium may have escaped by natural leakage. On the other 

 hand, there is a possibility, suggested by Prof. Joly, that an 

 absorption of radium may have taken place. This may easily have 

 exhausted its short life, leaving behind helium, which would then 

 be attributed to the radio-active constituents still remaining, and 

 thus give results in excess of the truth. 1 



This new method of investigation is of great promise, but a long 

 series of concordant observations will be required before we can 

 feel absolute confidence in its results. 



The Age of the Ocean. — We may now pass to the con- 

 sideration of the well-known method of Prof. Joly. This is based 



on the simple equation ^r— = x, where iS"a represents the quantity 



•of sodium contained in the Ocean, Na r the quantity annually dis- 

 charged into it by all the rivers in the world, and x the time which 

 has elapsed since the Ocean first came into existence. 



The quantity ]N"a is readily found from the volume of the Ocean 

 and its average chemical composition. The composition has been 

 determined with extreme accuracy, the analyses of Dittmar leaving 



1 Prof. Joly informs me of a case in which a specimen of pyromorphite was 

 shown to have absorbed radium, without uranium, to the extent of 10 centigrams 

 to the ton. 



vol. ixv. q 



