346 On the Electromagnetic Unit of Electricity. [June 21, 



rises from (f to 34°, the melting point of the decahydrated salt 

 Na 2 SO 4 .10H 2 O. Thereafter the solubility diminishes till the tem- 

 perature of 120° is reached. From 120° to 140° we find the change if 

 any is inappreciable, but at 160° a notable increase of solubility is 

 observed, which is still further increased at 180° and at 230°, the 

 highest temperature reached. 



Parts by weight of anhydrous sulphate of sodium dissolved by 100 parts 









of water at 









0°. 



34°. 



100°. 



120°. 



140°. 



160°. 



180°. 



230°. 



5 



78-8 



42 '7 



41 -95 



42-00 



42-9 



44-25 



46-4 



In view of these and other facts, theories of hydration can no longer 

 be admitted as competent to explain the act of solution in all or even 

 more than a few cases. 



X. " On the Determination of the Number of Electrostatic 

 Units in the Electromagnetic Unit of Electricity." By J. 

 J. Thomson, M.A., Fellow and Assistant Lecturer of Trinity 

 College, Cambridge. Communicated by Lord E-AYLEIGH, 

 F.R.S. Received June 19, 1883. 



(Abstract.) 



This paper contains an account of some experiments which have been 

 made during the last two years in the Cavendish Laboratory, Cam- 

 bridge. These experiments were made to determine "v" by com- 

 paring the electrostatic and electromagnetic measures of the capacity 

 of a condenser. The condenser consisted of two cylinders fitted with 

 guard-ring pieces. The electrostatic measure of the capacity was 

 calculated from the dimensions of this condenser. The electromag- 

 netic measure of the capacity was determined by a very slight modifi- 

 cation of the method given in § 775 of Maxwell's "Electricity and 

 Magnetism." In this method the condenser has to be repeatedly 

 charged and discharged by a commutator, and a very elaborate com- 

 mutator would be required to work the guard-ring part of the con- 

 denser ; for this reason the capacity of the guard-ring condenser was 

 experimentally compared with the capacity of another condenser 

 without a guard-ring, the capacity of the latter being altered until 

 the capacities of the two condensers were equal. The electromag- 

 netic measure of the capacity of the condenser without a guard- 

 ring was then determined by Maxwell's method. The ratio of the 



