Radium.'] SUB ANT ARCTIC ISLANDS OF NEW ZEALAND. 779 



transference through the drying-tubes from the stoppage of the boiling to the com- 

 mencement of observations in the electroscope being in every case fifteen minutes. 

 Readings of the position of the leaf were taken every two minutes for half an hour, 

 and the instrument was then recharged and the leak for the next half -hour similarly 

 determined. This was sufficient indication that the effect was that due to Ra emana- 

 tion. In some cases, however, the gas was studied for three or four hours ; but 

 although the leaks increased and the certainty was confirmed of the presence of 

 Ra, yet, owing to the active deposit, there was considerable time lost in bringing 

 the electroscope back to a normal condition of natural leak, and it was thus de- 

 cided to pump the gases out after an hour. 



The electroscope was charged to a potential of 220 volts, and the movement 

 of the leaf was read by a microscope and a micrometer eye-piece, so that the large 

 scale divisions could be subdivided into hundredths. For the means of its standard- 

 isation we are indebted to Professor Rutherford, who kindly sent us a solution con- 

 taining 3-14 X 10 ~^ gramme of Ra. Two separate eighths of this, each contain- 

 ing 3-925 xlO"^" gramme of Ra, were used as standards. Either of these treated 

 in the same way as the rock-solutions produced an initial leak of 64 micrometer 

 divisions per minute ; hence one micrometer division per minute corresponds to 

 6*13 X 10"-^^ gramme of Ra. 



"NATUEAL" LEAK. 



The determination of this is an important matter in an attempt to evaluate 

 with accuracy the amounts of Ra contained in a rock. In our case the electroscope 

 was always charged overnight — ^the insulation was sufficiently good for it to retain 

 a considerable portion of its charge for forty-eight hours — and the amount of the 

 leak determined. In general it decreased from the time of its initial charging the 

 night before to the introduction of the rock-gases. From a value of 1*5 per minute 

 the night before it became about 0*8 micrometer divisions per minute at the time of 

 introduction of the emanation. We consider that this latter number cannot be 

 more than 0*2 from the actual value during the examination of any of the rocks. 

 Such an error would lead to a wrong estimation of the amount of Ra in 20 grammes 

 of rock of 1-23 x 10"^^, or an error of 0-10 x 10^^" per gramme of rock. We therefore 

 consider that for the specimens we have examined the results may be relied upon 

 to about this extent. 



RESULTS. 



Bounty Island — 



No. 1. Granite . . . . . . . . 2-50 x lO-^' 



Auckland Island — 



No. 2. Trachyte, Musgrave Peninsula . . . . 2-10 . 



No. 3. Granite, „ . . . . 1-01 



No. 4. Pitchstone, „ .. ..1-90 



No. 5. Basic porphyrite, Adams Island. . . . 0*99 



No. 6. Basalt, „ (top) . . 0-81 



No. 7. Dolerite, „ „ . . 0-52 



No. 8. Diabase, Musgrave Peninsula . . . . 0-43 



No. 9. Gabbro, McClure Head . . . . 0-33 



