440 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



solution of potassa ; and as it, when pure, is a bad conductor, like water (554.), it was 

 occasionally improved in that power by adding sulphate of ammonia to it. But in all 

 the cases the effects were the same as before; decompositions of the same kind were 

 effected, and the electric current producing these was in the same direction as in 

 the experiments just described. 



935. In order to put the equal and similar action of acid and alkali to stronger 

 proof, arrangements were made as in fig. 8. ; the glass vessel A contained dilute sul- 

 phuric acid, the corresponding glass vessel B solution of potassa, P P was a plate of 

 platina dipping into both solutions, and ZZ two plates of amalgamated zinc con- 

 nected with a delicate galvanometer. When these were plunged at the same time 

 into the two vessels, there was generally a first feeble effect, and that in favour of the 

 alkali, i. e. the electric current tended to pass through the vessels in the direction of 

 the arrow, being the reverse direction of that which the acid in A would have pro- 

 duced alone : but the effect instantly ceased, and the action of the plates in the ves- 

 sels was so equal, that, being contrary, because of the contrary position of the plates, 

 no permanent current resulted. 



936. Occasionally a zinc plate was substituted for the plate P P, and platina plates 

 for the plates Z Z ; but this caused no difference in the results : nor did a further 

 change of the middle plate to copper produce any alteration. 



937. As the opposition of electro-motive pairs of plates produces results other than 

 those due to the mere difference of their independent actions (1011. 1045.), I devised 

 another form of apparatus, in which the action of acid and alkali might be more di- 

 rectly compared. A cylindrical glass cup, about two inches deep within, an inch in 

 internal diameter, and at least a quarter of an inch in thickness, was cut down the 

 middle into two halves, fig. 9. A broad brass ring, larger in diameter than the cup, 

 was supplied with a screw at one side ; so that when the two halves of the cup were 

 within the ring, and the screw was made to press tightly against the glass, the cup 

 held any fluid put into it. Bibulous paper of different degrees of permeability was 

 then cut into pieces of such a size as to be easily introduced between the loosened 

 halves of the cup, and served when the latter were tightened again to form a porous 

 division down the middle of the cup, sufficient to keep any two fluids on opposite 

 sides of the paper from mingling, except very slowly, and yet allowing them to act 

 freely as one electrolyte. The two spaces thus produced I will call the cells A and B, 

 fig. 10. This instrument I have found of most general application in the investigation 

 of the relation of fluids and metals amongst themselves and to each other. By com- 

 bining its use with that of the galvanometer, it is easy to ascertain the relation of 

 one metal with two fluids, or of two metals with one fluid, or of two metals and two 

 fluids upon each other. 



938. Dilute sulphuric acid, sp. gr. 1*25, was put into the cell A, and a strong solu- 

 tion of caustic potassa into the cell B ; they mingled slowly through the paper, and at 

 last a thick crust of sulphate of potassa formed on the side of the paper next to the 



