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Royal Society 



amount ; but there is a difficulty in bringing any large quantity of 

 it into contact at once with the silver and the dissolved salt. 



To facilitate this, we arrange that the silver plate should have 

 a horizontal position just under the surface of the liquid in the 

 cell ; and, in fact, we convert it into a small silver tray full of crys- 

 tals of the same metal which rise in projections to the very surface. 

 The copper plate lies horizontally under it, separated, if need be, 

 by a piece of muslin ; and connexion is made by a wire as usual. 

 The vertical part of the copper plate, from a little above the 

 liquid to the bend, should be varnished ; otherwise solution prin- 

 cipally takes place there, which causes the horizontal part of the 

 plate to drop off. Holes are made in the silver tray with the 

 view of shortening the communication between the air-surface and 

 the copper plate and of facilitating the movements of the salt in 

 solution. The solution itself may be contained in a shallow trough 

 or saucer, and the whole arrangement will be somewhat as in the 

 sectional view here given : — 



That dissolved oxygen is absolutely necessary for this chemical 

 change has been already shown ; but it was interesting to measure 

 by a galvanometer the difference of the currents obtained by means 

 of an ordinary, that is aerated, solution of copper nitrate, and one 

 from which the air had been separated to the greatest possible ex- 

 tent. A Thomson's galvanometer was employed, which had a 

 resistance of 2631*5 units at 18° # 3 C. Two cells were prepared 

 with vertical plates and alike in all respects, except that the one 

 contained an ordinary 6 per cent, solution of copper nitrate, and 

 the other a similar solution which had been deoxygenized by the 

 means described in our former paper. Another experiment was 

 made with a different pair of cells and an 11 per cent, solution. 

 It was necessary to use the 1-99 shunt ; and the following were 

 the amounts of deflection : — 



