RICHARDS, COLLINS, AND HEIMROD. — COPPER AND SILVER. 143 



The manipulation was simple. The platinum crucibles were cleaned, 

 dried at 160°, and weighed after two or three hours' cooling in a large 

 desiccator kept in the balance-room. In order to prevent leakage during 

 the electrolysis, the several stands were insulated by separate glass plates, 

 and all the connections were air lines. The apparatus was protected, as 

 in the earlier experiments with copper, by a miniature house with walls 

 of fine cotton cloth, which helped to exclude dust. When the current 

 was broken, the electrolyte was removed, the silver was rinsed twice with 

 water, a third filling with water was allowed to stand in the crucible for 

 two or three hours, and a fourth one remained in it over night. The 

 wash-waters were collected and filtered if the silver showed any tendency 

 to break off. In such cases a Gooch crucible was employed to collect 

 the particles ; and a very small filter, afterwards burned, served to catch 

 the minute flakes of asbestos detached from the mat. On the next morn- 

 ing, the crucibles were washed once more, rinsed twice with pure alcohol, 

 and finally dried and weighed as before. This method of treatment gave 

 opportunity for the diffusion of mother liquor from the intricate recesses 

 of the crystallized mass, while it did not run the risk of dissolving silver 

 which may attend the use of boiling water for washing. It is probable 

 that traces of water remained even after drying at 160"^, but the experi- 

 ments on page 139 show that these traces were less than one part in ten 

 thousand. 



Weighings were made upon the balance which served for the weigh- 

 ings in the earlier work upon copper, — one which has served also for 

 many determinations of atomic weights. Its results with small objects 

 may be depended upon to within ^ milligram. All weighings were 

 made by double substitution, a similar vessel being used as a tare, and 

 the weights were of course carefully standardized. Since the question 

 concerned merely the comparison of silver with silver, the results were 

 not corrected to the vacuum standard. 



The object of the work described below was to determine the relative 

 values given by the silver voltameter under difi^erent conditions, and to 

 interpret the results in such a way as to point out the true value. For 

 such a purpose it is convenient to select some one method as a standard 

 of reference ; and because the porous cup method, conducted at 20°, soon 

 showed itself to be capable of giving the most constant results of all the 

 various modes of procedure, it was chosen as this standard. For 

 example, in one case a given current caused depositions of 2.4o744 and 

 2.43749 grams of silver in two cells placed in series, — a difference of 

 only 2 parts in 100,000. 



