374 RECENT PROGRESS IN PHYSICS, 



cell with a porous diapliragm, and found, after the evolution of a 

 certain quantity of gas representing one equivalent of water, on the 

 side of the positive pole about one-fourth an equivalent of sulphuric 

 acid more, and on that of tlie negative pole one-fourth an equivalent 

 less than before the experiment. In this case, therefore, besides the 

 decomposition of water, a separation of water from sulphuric acid had 

 taken place analogous to the decomposition of the salt, though only 

 to the amount of one-fourth an equivalent. 



This transmission of sulphuric acid to the amount of nearly one- 

 fourth an equivalent of the deconaposed water was independent of the 

 degree of concentration of the acid. Daniell modified the experiment 

 in various ways, but always obtained the same result. 



A voltameter filled with sulphuric acid, therefore, is not applicable 

 for measuring the force of current passing through the solution of 

 sulphate of soda. 



Besides the double cell with the solution of the salt, Daniell then 

 introduced, in addition, fused chloride of lead in the circuit of 

 the battery. It was placed in a bent tube of (green) glass, free from 

 lead, above a spirit lamp ; a platinum wire in the fused chloride of 

 lead served as the negative pole, while the positive was formed of 

 plumbago. At the latter chlorine gas was evolved ; upon the former 

 the reduced lead was deposited. 



Here the following remarkable result was obtained : For one equiva- 

 lent of chloride of lead decomposed, in the other cell one equivalent 

 of water, and in addition to this one equivalent of salt were decom- 

 l)osed. According to the usual acceptation, therefore, it would appear 

 that in the cell with the solution of salt there was a chemical action 

 twice as great as that produced by the same current in the cell with 

 the chloride of lead ; this is in direct contradiction to the law of con- 

 stant electrolytic action. 



In order to avoid this contradiction, Daniell considers the consti- 

 tution of salts from a quite novel point of view. He considers, for 

 instance, the sulphate of soda not as immediately formed by the com- 

 bination of acid and base, but assumes the electro-negative constituent 

 of the salt to be one equivalent of sulphur with four equivalents of 

 oxygen, while the positive is only sodium. 



According to this view, therefore, 



Sulphate of soda is not SO,. Na.O, but SO^. Na. 

 Sulphate of potassa is not SO3. KO, but SO^. K. 

 Nitrate of potassa is not NO5. KO, but NOg. K. 

 Phosphate of soda is not P0|. Na.O, but POy. Na. 

 Sulphate of copper is not SO3. Cu.O, but SO4. Cu. 



If the current pass through the solution of such a salt it is not the 

 water but the salt which is decomposed into the constituents just 

 named. In the electrolysis of the sulphate of soda, therefore, SO4 is 

 produced at the positive pole as a direct etfect of the electrolysis, from 

 which (by a secondary action) is immediately set free, leaving SO3 ; 

 while at the other pole the Na. separated takes oxygen from the water, 

 and in this way liberates for each equivalent of Na. an equivalent of 



