ON ELECTROLYTIC METHODS OF QUANTITATIVE ANALYSIS. 245 
The measurements of currents for most purposes is effected by the 
usual gravity ammeters indicating 0°5 to 2 amperes, but much smaller 
currents than 0°5 ampere must be used in bismuth estimations ; the 
most satisfactory mode of measuring them was by means of a delicate 
astatic galvanometer, with silk fibre suspension, and carefully calibrated 
in the position in which it was used. A curve was plotted for converting 
deflections into amperes, so that 0°01 ampere could be read directly, and 
0-001 ampere current measured by interpolation, Provision was also 
made for shunting in a gravity voltmeter. 
For bismuth estimation a box of coils giving high resistances was used 
for regulating the current, as the platinoid spiral with slide, in genera’ 
use in the laboratory, did not cffer more than 30 ohms resistance. 
A long series of preliminary experiments made with bismuth nitrate 
solutions and different forms of electrodes indicated :— 
1. That the conical form of negative electrode is not suitable for use 
in the estimations of bismuth, as it is difficult to get good adherent deposits 
on the cones, unless the solutions are very dilute. 
2. That the large, smooth surface of a carefully spun platinum dish is 
hest suited for the negative pole in bismuth estimations. The dishes used 
in the test estimations weighed from 35 to 38 grammes, and the internal 
areas averaged 190 square centimetres. 
3. That a large flat spiral of platinum wire for the positiye electrode is 
more satisfactory than a disc, as it allows free circulation in the liquid. 
A large number of experiments were made in the first instance with a 
view to ascertain the kind of solution most convenient for electrolysis, 
the nitrate or sulphate being used as the starting point in the production 
of the different liquids actually electrolysed. Irregular results were 
obtained with the simple nitrate containing varying proportions of free 
nitric acid ; but good determinations were more easily made in solutions 
of the sulphate when electrolysed by currents of 0-08 to 0:2 ampere. 
On the other hand, for the purposes of actual analysis, it seemed of 
more practical importance to ascertain how far the more convenient 
nitrate solution could be made to afford good reguline deposits of bismuth, 
which should admit of washing without loss or material oxidation. It 
was found that careful, adjustment of the current is one of the elements 
of success, but not the only one, and that 0:03 ampere is the maximum 
current that should be used nearly to the end of the operation, though 
0:1 may be passed to complete deposition. Regulation of the current did 
not alone prove sufficient to neutralise the effect of excess of nitric acid, 
and most of the substances were tried which have been recommended for 
use in similar electrolyses. Of these, metaphosphoric acid and citric acid 
proved so much better than any others we employed that our attention 
was directed chiefly to examine their effects in test experiments with 
bismuth nitrate. 
Test EXPERIMENTS. 
The bismuth used in the preparation of the test solutions was carefully 
purified. In the first instance it was repeatedly fused with small quan- 
tities of nitre and cast. This metal was then heated to low redness for 
fifteen minutes with potassium cyanide and sulphur, with constant 
stirring ; was again cast, and reheated to bright redness with 5 per cent. 
of a mixture of pure sodium and potassium carbonate. The fine metal so 
obtained was dissolved in nitric acid, the solution evaporated to a small 
bulk, and then precipitated as oxynitrate by water. The washed pro- 
