﻿the Rocks of the St. Gothard Tunnel. 205 



The expelled gases were passed through soda-lime and the 

 tinabsorbed gas received in a rubber bag, from which they 

 were finally drawn into the exhausted electroscope : on the 

 way passing again through the soda-lime tube. This is 

 advisable, seeing that much of the emanation is probably 

 retained in the tube into which it is borne along with the 

 C0 2 expelled during ihe decomposition of the rock. Awash- 

 out of the furnace-tube is finally effected by completing the 

 filling of the electroscope through the entire system of 

 furnace-tube and soda-lime tube. The chemicals used in 

 this process were separately tested for radium emanation in 

 the manner described, loc. cit. p. 147. 



The electroscopes used were made of glass u bolt-heads," 

 having a voluminar capacity of 500 c.c. The sulphur insu- 

 lation was carefully shielded, by black paper tubes, from the 

 light ; past experience having showed that otherwise error 

 might be occasioned. After a preliminary exhaustion and 

 filling with air from the laboratory the rating of the move- 

 ment of the gold leaf was carried on for about one hour before 

 the admission of the gas, or for such a time as was necessary 

 for the normal low reading of about 7 scale divisions per 

 hour to be attained. If a higher reading — say, 8 to 12 scale 

 divisions — persisted with no sign of diminution, this higher 

 reading was subsequently deducted from the final readings. 

 The filling from the furnace was effected through a capillary 

 tube, which regulated the rate of inflow so that 8 minutes 

 were required for the complete filling of the electroscope. 

 The drying material used was phosphorus pentoxide, and a 

 plug of tightly compressed cotton-wool was interposed between 

 the drying-tube and the electroscope. 



The determination of the constant, or calibration of the 

 electroscopes, was effected as described, loc. cit. pp. 142, 146. 

 There was some uncertainty about the constant of one of 

 the electroscopes. Readings were obtained ranging from 

 0-6 x 10" 12 to 08 x 10- 12 . 1 am inclined to the view that the 

 last value is the best supported ; however, as comparative 

 observations, which I effected by admitting to each electro- 

 scope the same volume of ground gas (drawn from a depth of 

 1 metre in the College Park), showed but little difference in 

 the effects produced on the two instruments, and as the one 

 exhibited very steadily 0*6 x JO -12 as the calibration value of 

 a gain of 1 scale division per hour, I have thought it best to 

 give the determinations made by the second instrument in 

 terms both of the high and the low constant. The more 

 important determinations applying to the Finsteraar granite 



