48 Dr. H. E. Watson and Mr. Gostabehari Pal on 



nitrate similar to the others, but in which the apparatus was 

 filled with hydrogen instead of air, a leak of 36*5 scale- 

 divisions per hour was observed, while the same weight of the 

 salt produced a leak of 133*5 scale-divisions per hour in air. 



Using these data, and assuming that the rate of leak is a 

 linear function of the quantity of hydrogen (a doubtful 

 assumption), it maybe calculated that if 100 c.c. of hydrogen 

 are introduced into an electroscope of 500 c.c. capacity, a 

 leak of 15 divisions per hour would be reduced to 12*7 and 

 one of 10 to 8*4. With a natural leak of 5 it will be seen 

 that the error in the latter case is nearly 100 per cent., and 

 100 c.c. is not at all an unusually large quantity of hydrogen 

 to be evolved. 



As the actual leaks increase in magnitude the relative 

 error decreases, but becomes by no means negligible. 



In consequence of this, all the experiments which had 

 been done were repeated with a tube of red-hot copper oxide 

 inserted between the heating flask and the first tap. The 

 effects of this were very satisfactory, as the pressure never 

 rose above half an atmosphere during the fusion ; and after 

 cooling the flask, it was rarely 30 mm. more than the initial 

 pressure. Even if this were due entirely to hydrogen which 

 was not absorbed in its subsequent passage over the copper 

 oxide, the amount (10 c.c.) would be insufficient to influence 

 the results. 



It was found best to keep the electroscope continuously 

 charged ; consequently at the end of each day's readings it 

 was evacuated to a pressure of about 10 mm.; and if the 

 rock which had been examined had contained more than the 

 usual excessively small quantity of radium, air was again 

 admitted, and pumped out. At this pressure the leak was 

 only 0*3 division per hour, and consequently the fibre would 

 remain on the scale for a long period. Immediately after 

 the electroscope had been filled with fresh emanation, it was 

 charged to a higher potential, so that the pointer reached a 

 spot about 15 divisions off the scale, and no readings were 

 taken for at least an hour (except in the case of large leaks). 

 The reason for this was that the leaf fell very rapidly at first 

 if charged up directly from zero potential owing to some 

 " soaking in " effect of uncertain duration. 



The sensitiveness of the electroscope remained practically 

 constant over the whole scale, as might be expected from the 

 potential to which it was charged ( — 300 volts) ; but the final 

 values were as far as possible deduced from readings over the 

 same part of the scale owing to a slight variation in parts 

 which was evidently due to rigidity of the leaf. 



