302 The Radioactivity of Sea-Water. 
I think, no room to suppose that an unsuspected source of 
radium contamination can exist. This being so, the principal 
question arising is whether such measurements truly re- 
present the whole of the radium present. It is indeed 
remarkable that the quantities of emanation dealt with are 
by any process capable of extraction from the large volumes 
of highly saline liquid involved. Thus Ramsay and Soddy* 
estimate that the volume of emanation in equilibrium with one 
gram of radium is but one cubic millimetre; a number closely 
agreeing with Rutherford's calculated value f. It follows 
that in about a litre of sea-water there is a maximum of 
about twenty billionths of a cubic millimetre. That such a 
minute quantity can be extracted with such an approximate 
agreement among observations is remarkable. On this point 
I thought it of interest to see if I could extract with accuracy 
a known small quantity of emanation generated in the sea- 
water. I accordingly introduced info one of the flasks con- 
taining 1400 c.cs. of sea-water, after the determinations were 
concluded, one milligram of uraninite in solution. A few 
hours afterwards the sea-vs ater was boiled in the usual manner 
and the emanation transferred to the electroscope. The yield 
was almost exactly correct ; the small discrepancy (on the 
side of excess) being no more than a small experimental 
error would account for. This experiment, indeed, does 
not prove that after prolonged ebullition there may not 
remain over a residuum of emanation in sea-water, but it is 
an assurance as to the reality and meaning of the positive 
values observed. 
It appears to me that the above results go some way 
towards explaining the difficulty which Eve experienced in 
accounting for the amount of ionization observed over the 
ocean (loc. cit.). It must be remembered that over a fluid 
medium emanation may be derived from considerable distances 
below the surface, not only by convection currents bringing 
fresh portions of the medium to the surface, but by the extrac- 
tion of emanation whenever air becomes mixed up with the 
water. The process, in the latter case, being in fact the same 
as that frequently used in extracting emanation from liquids. 
It seems as if the atmosphere over the ocean might draw upon 
considerable depths of water for supplies of emanation, 
whereas over the land the emanating part of the radium 
must be practically only those atoms at or Aery near the 
surface of exposed solids. 
Oceanic radioactivity is, most probably, in part referable 
* Proc. Rov. Soe. lxxiii. no. 494, p. 346. 
t ' Nature/ Aug. 20, 1903. 
