r 34 
As to the nature of the gas evolved, it was found to suffer 
no diminution in volume when agitated with caustic potash 
solution or with alkaline pyrogallate. It was not therefore 
either carbon dioxide or oxygen. With the kind assistance of 
Mr. J. B. Allen an attempt was made to prepare a spectrum tube 
of the gas, but in the absence of suitable tubes all our efforts 
resulted in failure. Whilst positive evidence of identity is lacking, 
there can be no doubt that the gas was wholly or almost wholly 
helium, a gas given off in greater or less quantity from all radio- 
active minerals when suitable means are taken to release it. 
The greatest interest attaches to the accurate determination 
of the helium in radioactive minerals since recent researches, par- 
ticularly those of Rutherford and Strutt, have indicated that 
helium may by its slow accumulation be acting as a chronometer 
of geological jime. The assumptions on which the calculations 
are based are : — 
(1) Uranium, radium, thorium and other associated radio- 
active matters are producing helium during thier 
atomic disintegration at a very slow' rate which has 
already been measured with some degree of accuracy ; 
(2) This production of helium wotdd begin in a radioactive 
mineral immediately it consolidated from aqueous 
solution or igneous fusion ; 
(3) Only negligible quantities of helium would be able to 
escape from the mineral until disturbed by man, by 
far the greater part being occluded in the mineral in 
masses approximating atomic dimensions, and under 
conditions of great condensation. Leakage of helium 
commences immediately the minerals are liberated 
from the enclosing rock.* 
Working on the mineral zircon from lava of known ages, 
Struttf showed that the ratio of helium in cubic centimetres to 
uranium oxide in grammes was a function of the age of the 
mineral. In calculating this ratio thoria is reckoned as equivalent 
in helium production to one-fifth its weight of uranium oxide 
(U 3 O s ). This ratio was found to be less than unity for rocks of 
Tertiary age, between 10 and 30 lor rocks of Palaeozoic age, and 
over 50 in the case of a single rock of Archaean age. The “ Helium 
ratio ” for this Pilbarite is 2.9, which would indicate that this 
mineral was evolved from its parent in Post-Palaeozoic times, if 
all previously accumulated helium be assumed to have been 
liberated during the hydration and possible oxidation which then 
took place. 
Strutt, Proc. Roy. Soc. Jan. 1909. 
t .. „ Dec. 1909. 
