On the Electrolysis of Silver and of Copper. 403 



have become completely coated with a thick green coating, 

 when it gradually ceases. All the phases of this action are 

 illustrated in curves 1 and 4 ; the first part, namely the loss 

 of weight at the beginning, is not shown in curves 7 and 10, 

 as the plates had already gained weight before the first 

 weighings were made. 



Electro-chemical Equivalent of Silver. — Some preliminary 

 experiments were made on this subject, but owing to faults 

 being discovered in the standard galvanometer the experi- 

 ment was put aside and has not yet been resumed. The first 

 trouble that was experienced with the galvanometer is per- 

 haps worth recording here. It was a new instrument of the 

 sine type, made almost wholly of brass, and had a short 

 needle suspended at the centre of the coil, and a lighHndex 

 enclosed in a long rectangular box, the sides of which were 

 made of thin brass, and the top and bottom of glass. The 

 coil was forty centimetres in diameter, and consisted of one 

 layer of silk-covered copper wire containing seventy turns, 

 making up a breadth of about five centimetres. The constant 

 of a second and more convenient instrument, of high sensi- 

 bility, was determined by comparison with the sine galva- 

 nometer, and then used in accordance with the method 

 described below for measuring the current passing through 

 two silver-silver-nitrate electrolytic cells arranged in series. 

 The magnetic field at the needle of this auxiliary galvano- 

 meter was produced by permanent magnets, and was so in- 

 tense as to be hardly at all, certainly not to the extent of 

 T ^ th per cent., influenced by variations of the Earth's 

 magnetism. In the first three trials the constant of the 

 second galvanometer was such that the current produced a 

 deflection of about 20° on the sine galvanometer, an angle 

 which could be measured with a fair amount of accuracy, but 

 which of course did not take full advantage of the sine prin- 

 ciple. These three measurements each gave the same result, 

 namely '0011185 gramme as the amount of silver deposited 

 by a coulomb of electricity. The constant of the second gal- 

 vanometer was then changed so as to give a deflection of 

 about 40° on the sine galvanometer, and then a difference 

 amounting to about one sixth per cent, was discovered be- 

 tween the value now obtained and that previously got for 

 silver. This seemed to indicate a departure from the law of 

 sines, and suggested, as almost the only possible explanation, 

 magnetic brass. A small Bottomley's reflecting-magneto- 

 meter was then set up, and the magnetic field at its needle 

 made almost zero by means of a permanent magnet. The 

 brass box enclosing the needle was unscrewed and, the needle 



