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the framework with the three plates. In addition to the voltameter, the jar contained a small mercury gauge. The upper rim of the jar was ground, and previous to exhaustion was coated with grease, upon which pressed a ground-glass plate. The current was derived from storage cells, and was measured by a tangent galvanometer or a Thomson's magnetostatic centiam pèremeter; it was adjusted to the proper value, and kept constant throughout an individual experiment by having in circuit a carbon rheostat and a set of specially prepared resistances of different values. The copper plates were made of commercial sheet copper (24 B.W.G.): they were cleaned with emery paper, nitric acid, caustic potash, tap water, and, finally, distilled water, and a deposit was laid on them before being weighed for an actual experiment. The deposit was treated in the way recommended by Gray; I found it advantageous to have the distilled water hot, as on removing the plate the deposit dries more quickly, and consequently the possibility of oxidation occurring is diminished. The plates, after being in the balance case for a variable period— never less than three hours, generally over night-were weighed on a 16-in. Oertling balance, which weighed accurately to 1 tenth-milligram. The solutions were made by dissolving ordinary commercial sulphate in distilled water and filtering; the solutions with no free acid added were used immediately, so as to conduct the electrolysis before the formation of basic salts; the acid solutions contained 1 per cent. free sulphuric acid. When all the joints were air-tight, by being closed with cement, no difficulty was experienced in retaining the partial vacuum constant throughout each individual experiment. Leakage of the current at either voltameter was tested for throughout the experiments, and this point was considered satisfactory whey, both voltameters being under atmospheric pressure, the deposits did not differ by more than 1 or 2 tenth-milligrams. Particular attention was paid to the handling of the plates before and after each experiment, especially in the replacing in, or removal from, their clips. By means of a small rod, acting as a lever, friction was prevented between the clips and plates, and hence the slight danger of loss of copper prevented. Immediately after the current was stopped in each experiment, the framework holding the plates was quickly removed from the solution and plunged into distilled water so as to remove all solution from the plates-oxidation occurs very rapidly if any solution remains on the plates, even for a very short time.

My first object was to ascertain if any difference in the weight of the deposit was caused by placing one voltameter in vacuo, without any special regard to the current density (which throughout all the experiments was the same at both cathodes). The experiments arranged in Table I show an appreciable, though inconstant, differ

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0.9

14.6

39

0.75

0.6849

0.6844

0.0005

+

7

Some vacuum de

posit lost.

Mar. 17

1070

18.

1110

0.8

15:4

35

0.62

0.5600

0:5589

0.0011

+ 18

22

1095

0.7

18.0

45

0.62

0.6345

0.6310

0.0035

+ 55

The solutions in each experiment contained no free acid, and were not previously used in any experiments.

ence.

In ten experiments, with the sulphate solution containing no free acid, nine gave a greater deposit in vacuo than in air, one giving a negative difference. In this experiment (February 18) the deposit was not of the usual colour, and it is possible it was oxidised. Great care was necessary in washing the vacuum deposit, as often the copper was deposited in a loose form at and near the edges; in the experiments of March 17 and 18 a little of the vacuum deposit was lost in drying between the folds of blotting paper. It was not to be expected that the experiments in this table would show very concordant results; as, even presuming that the deposit in vacuo is constant, the inconstancy of deposits in air from neutral solutions would cause an inconstant difference.

In order to get a constant deposit in air, I next added acid to the air solutions and compared the deposits from these with the deposits from a neutral solution in vacuo.

Table II includes experiments conducted in this way: in the experiments of February 24 and 29 some of the vacuum deposit adhered to the blotting paper and was lost. I cannot account for the negative result of February 25. The remaining experiments show fairly concordant differences. It will be observed from Tables I and II that, with the exception of those experiments in which some of the deposit was mechanically lost, the percentage difference is greater with acid solutions in air than with neatral solutions, which agrees with Gray's observations that the deposit (in air) in neutral solutions is generally higher than in acid solutions.

I have brought together the experiments included in Table III, although the results are very inconsistent and puzzling. It will be seen that, if the neutral solutions have been used in previous experiments and acid added to them for a fresh experiment, the difference between the weights of the two deposits varies not only in amount but also in sign. An explanation of this may be that electrolysing a neutral solution changes its chemical constitution, and that adding acid afterwards does not remove all basic salts. Accepting this possible explanation, I abandoned for the present using neutral sulphate in either voltameter. In all the remaining experiments described in this paper, both solutions were freshly prepared, not previously used in any experiment, and contained 1 per cent. free sulphuric acid. The results I have obtained in this way are more concordant and very interesting. A deposit obtained from a neutral solution in vacuo shows the same looseness in deposition as in air; but the addition of a little free acid causes the copper to be deposited in a compact form, which is not liable to be lost in the washing or drying.

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Both solutions freshly prepared for each experiment; air solution with 1 per cent. free acid, vacuum solution neutral.

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The solutions were used previously without any free acid; 1 per cent. acid added in these experiments.

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