KECENT PROGRESS IN PHYSICS. 



373 



In the circuit of a battery of thirty constant zinc-copper elements 

 a decomposing cell of the following construction was inserted. A 

 strong glass cylinder of ahout 14 inches cubic content and closed at 

 both ends, was cut lengthwise and a porous ^.^ ^^g 



clay plate inserted between the two halves ; the 

 whole was then connected again by means of 

 brass rings with clamp screws. In each of 

 the two cells thus formed 'a platinum electrode 

 2|- inches long and 1 inch wide was fastened 

 to a wire passing through the bottom of the 

 vessel. A bent glass tube was ground to the 

 top of each cell for the purpose of conducting 

 off the evolved gas. A preliminary experi- 

 ment showed that after 24 hours only a very 

 minute quantity of distilled water passed from one cell into the other. 

 Figure 156 is a sketch of this apparatus. 



The first experiment with this apparatus was the following : Both 

 cells were filled with a solution of sulphate of soda so that the 

 electrodes were just covered ; the apparatus was then kept in connexion 

 with the battery until the evolved gases amounted to 29 cubic inches, 

 (corrected for pressure and temperature 28.3 cubic inches.) There 

 were, therefore, 3.6 grains of water decomposed. 



The solution at the negative pole was strongly alcaline ; it was 

 carefully drawn oft with a glass syphon, and saturated with sul- 

 phuric acid of known strength, and was thus found to contain 12 

 grains of free soda. The solution at the positive pole was acid, and 

 by saturating it with a solution of carbonate of soda 15.1 grains of 

 free sulphuric acid were found. 



If, now, the atomic weight of oxygen be taken at 100, that of water 

 will be 112.5, that of sulphuric acid 500, and that of soda 390. 

 Hence result the follov/ing proportions : 



112.5 : 390 = 3.6 : 12.5 

 and 112.5 : 500 = 3.6 : 16 



For one equivalent of the water decomposed, therefore, in one of 

 the cells, nearly one equivalent of sulphuric acid has been separated, 

 and in the other nearly one equivalent of soda. 



Several experiments with other salts gave the same result. But 

 this experiment furnishes no datum for determining the relation of 

 the chemical decomposition to the force of current. This relation 

 would have been simply and precisely ascertained if, in addition to 

 the decomposing cell, a tangent compass had been inserted in the 

 circuit. It is to be regretted that this instrument appears to be so 

 little known in England, or at least very little use is made of it. 

 Daniell inserted into the circuit, instead of it, a voltameter filled with 

 dilute sulphuric acid, and found equal quantities of detonating gas 

 evolved in the voltameter and in the cell with sulphate of soda ; but 

 it appeared that, besides the decomposition of water, still another 

 effect had taken place in the voltameter, for the acid had accumulated 

 at the positive pole. For this reason Daniell examined more accu- 

 rately the decomposition of dilute sulphuric acid in a decomposing 



