the Rocks of the Transandine Tunnel. 



41 



(4) Standardized from the acid solution of No. 5 of the 

 Andes rocks. This contained 9'17 x 10~ 12 gram radium. To 

 this was added 7*07 x 10 -12 gram radium from standard 

 solution. Closure 21 days. 



Total radium = 16*24 x 10" 12 gram. 

 Gain = 19 scale-divisions per hour. 

 16-24x10- 



Hence 







19 



= -85 x 10- 



The use of the slow admission capillaries used in the 

 foregoing experiments was continued throughout the rock 

 tests, although comparative experiments in which the ad- 

 mission was made as rapid as was consistent with the safety 

 of the gold leaf, and again with the capillary tube, showed 

 no detectable difference. The construction of the electro- 

 scopes were of course also preserved in all particulars alike. 



The importance of conducting the calibrating experiment 

 under conditions of the solution as nearly identical as 

 possible with those under which the actual experiments are 

 made, seems immediately apparent from these results. Such 

 a variation in the constant as appears in the above experi- 

 ments, in the case of electroscope A, might introduce an 

 error of deficiency of as much as 30 per cent, into each 

 experiment. 



Effect of Sunlight. 



Care was taken to shield the electroscopes from bright 

 daylight, and the discharge rates were read throughout in 

 semi-darkness. A short series of observations showed that 

 the discharge rate might be raised from 5 scale-divisions 

 per hour — the normal leak — to over 80 per hour in direct 

 sunlight, and 30 per hour in bright diffused light. The 

 normal leaks of 10 and 5 scale-divisions per hour for " A " 

 and " B " respectively, were wonderfully constant from day 

 to day, scarcely ever varying over 1 scale-division per hour, 

 three hours after refilling with fresh air. Under these 

 circumstances an increase of 2 or 3 scale-divisions per hour, 

 at an interval of three hours subsequent to the introduction 

 of the emanation, was unmistakably evident. 



This would be accounted for by as small a quantity as 

 3 x '8 x 10~ 12 gram of radium in a solution containing say 

 10 grams of rock. Hence a quantity of radium of about 

 *24xl0~ 12 gram per gram was capable of measurement 

 with a fair degree of accuracy. 



The method of extraction of the emanation was essentially 

 that described by Professor the Hon. R. J. Strutt, with the 



