The EARTH’S MAGNETIC FIELD IN INTERNATIONAL UNITS. 
451 
were calibrated at the Standards’ Office, and the author’s best thanks are due 
to Mr. Chaney for his kindness in supplying him with the corrections.'^' The 
method of substitution was employed in the weighings, two auxiliary sets of weights 
being used, one to counterpoise the empty crucible, and the other to counterpoise the 
silver. Since the weights employed to weigh the silver consisted of a brass one- 
gramme weight, and 0'64 gramme in platinum, the mean density of the weigiits was 
10 ’6. The density of silver being 10 ’4, no correction had to be applied on account 
of the air displaced to reduce the weights to vacuo. 
The time during which a deposition lasted (2 hours) was taken with a chronometer, 
the rate of which was determined each day by means of time signals from 
Greenwich. 
KAHLEf having shown tliat the weight of silver deposited depends to a slight 
extent on whether the deposition takes place on a platinum surface, or whether there 
is already some silver in the hasin, it was decided to always start with the basins 
free from silver, as in this way identical conditions in the various measurements can 
most easily be secured. The silver deposit was first rinsed with distilled water three 
times, the water being left in the basin for about 10 minutes in each case. The 
hasin was then allowed to soak in water for at least 3 hours. It was then dried by 
being heated for about 10 minutes over a spirit lamp and placed in a desiccator to 
cool. The hasin was left in the balance case, in which was jjlaced a dish of calcium 
chloride, for at least half-an-hour before the final weighing. In order to make 
certain that the silver nitrate was completely removed from the deposited silver, the 
water in which the basin had soaked for 3 hours was always tested with dilute 
hydrochloric acid. If no milkiness was produced, it was assumed that the deposit 
was free from the salt. 
A consideration of the numbers given in the table on p. 453 will show that the 
weight of silver obtained in the porous pot voltameter is less than that in the Hayleigh 
voltameter, the mean difference being 0'43 milligramme in U638 grammes, or 
0'026 per cent. This is considerably less than the difference (0'082 per cent.) 
obtained by PtiCHARDS, Collins, and Heimrod. These experimenters do not state 
in their paper, whether the solution they used in the Rayleigh form of voltameter 
was quite unused. If it had been used several times, the high value they obtained 
might be accounted for. In the five consecutive experiments they quote, the excess 
of the Rayleigh over the porous pot deposits amounts to 0’074, 0'080, 0'090, 0‘094, 
0'072 per cent. With the exception of the last value there seems some evidence for 
a steady rise, as though the solution employed were gradually getting aged. 
On the whole, the weights of silver obtained with the porous pot voltameter are 
slightly more concordant than with the Rayleigh form, but there does not seem much 
* The weights used have also been compared with a 1-gramme weight, the value of which has been 
supplied by the International Bureau at Sevres, Jan., 1902. 
t Loc. cit. 
3 M 2 
