176 Scientific Proceedings, Royal Dublin Society. 



Capacities may, of course, be compared by the electrometer 

 without any use of radio-active substances by charging the un- 

 known capacity to a potential which is measured by the elec- 

 trometer, and then sharing the charge with a known capacity, and 

 again measuring the potential. The method of working is not, 

 however, as accurate as that described above, especially when the 

 capacities are small. 



Capacities are often compared by charging them to the same 

 potential, and discharging them through a ballistic galvanometer. 

 The galvanometer deflection must be accurately read, and a cor- 

 rection applied for damping — observations which cannot be made 

 with the accuracy with which we can compare two intervals of 

 time. Again, when the capacities are small, they must be charged 

 or discharged through the galvanometer a great number of times per 

 second, which requires carefully constructed apparatus to enable the 

 number of charges to be accurately known. In addition, it is 

 somewhat difficult to be certain that the apparatus is working 

 properly ; for example, an error might arise through faulty 

 insulation, and escape detection. 



The method of De Sauty is free from many of the objections 

 mentioned above ; but others might be urged against it, and espe- 

 cially that it can be of little use when the capacities are very 

 small. 



One of the chief advantages of the method described in this 

 paper is that, from the nature of the apparatus used, it is scarcely 

 possible for any serious source of error to come in without detection ; 

 a faulty insulation, for example, can easily be guarded against. 

 The only quantity requiring to be measured is an interval of time, 

 which can be done with great accuracy. The ionisation produced 

 by the uranium keeps very constant througho ut the time required 

 to make a determination; and there is no other quantity that 

 requires to be kept very constant. The potential of the battery 

 joined to one of the plates between which the uranium is placed 

 may vary considerably between the observations, and produce no 

 effect, provided the potential is sufficiently great. 



The only objection that seems likely to be made to the method 

 is the fact that it employs a quadrant electrometer, the use of 

 which in ordinary laboratory work has hitherto been discouraged. 

 As stated above, the writer sees no reason for the reluctance to use 



