THE EAETH’S MAGNETIC FIELD IN INTERNATIONAL UNITS. 
457 
The most probable value for the electro-chemical equivalent of silver can hardly l)e 
obtained by taking a mean of these numbers, for not only is the accuracy attained in 
some of the measurements much greater than that attained in the others, but also 
the condition of the silver nitrate solution employed was diflPerent in the different 
cases. The measurements numbered 1 and 4 in the above table may at once be 
neglected, for not only do the values obtained indicate that the accuracy of the 
measurements was not very great, but also this ojiinion is confirmed by a study of 
the original papers. Of the remainder, numbers 5 and 6 are the only ones in which 
any information is given as to the state of the silver nitrate solution. Rayleigh and 
Kohlrausch used neutral solution which had not received any special treatment, but 
they do not give any information as to the amount of silver which had been 
deposited from each cubic centimetre of the solution,"^ and so we are not able to tell 
whether the solution was fresh, i.e., had lost very little silver, or whether it had 
already been electrolysed to such an extent as to cause the weight of silver deposited 
to be greater than the normal. Patterson and Guthe treated their solution with 
silver oxide, since they found that the weight of silver deposited by a solution 
treated in this manner was more constant than in the case of an untreated solution, 
and also with a view to insuring that the solution should always be neutral. They 
did not, however, use a fresh solution for each deposition, the used solution being in 
each case returned to the stock bottle, and no mention is made of the quantity of 
silver which had been deposited before the measurements recorded in their paper. 
There is no doubt, however, that their solution had been used considerably. Now, 
KahleI has shown that the treatment of silver nitrate solution with silver oxide 
causes the weight of silver deposited by one coulomb to increase by 5 parts i]i 
10,000. He bas also shown that the addition of silver oxide does not do away witli 
the rise in the weight of the deposit as the solution is used. Its effect, as indeed is 
the rate at which the rise itself takes jilace, is quite irregular. On account of the 
treatment with silver oxide, the solution employed will certainly give a value for the 
electro-chemical equivalent which is liigher than the normal. Making use of 
Kahle’s value for the effect of the treatment with silver oxide, the value which 
Patterson and Guthe would have obtained with untreated solution would be 
0'0011186. Even this value is, on account of the solution employed being old, 
probably considerably higher than would have been obtained by these observers, if 
they had used a fresh solution. 
Kahle did not directly measure the electro-chemical equivalent, but he measured 
by means of an electro-dynamometer the E.M.F. of a Clark cell, and he then used 
this cell in conjunction with the same resistance coils which had been used in the 
previous experiment to measure the current which he sent through the sih er 
* The quantity of silver nitrate in the solution remains, of course, unaltered, l)irt the above is a us(4ul 
measure of the amount a solution has been electrolysed. 
t ‘ Brit. Assoc. Rep.,’ p. 148, 1892, and ‘ 5Vied. Ann.,’ vol. G7, p. 1, 1899. 
VOL. CXCYIII.—A. 3 N 
