of Silver and of Copper. 399 



manner, and allowed to soak for about fifteen minutes before 

 being dried. It is important to bear in mind that the water 

 used for the first rinsing and washing must be clean water 

 which has been recently distilled, and which has not had an 

 opportunity of absorbing any impurity, such, for example, as 

 a minute quantity of common salt, by coming in contact with 

 anything which has not been recently and thoroughly 

 cleaned. The water supplied by the Glasgow Corporation for 

 domestic purposes, although very nearly pure, clouds imme- 

 diately on contact with silver nitrate, and throws down a con- 

 siderable precipitate of silver chloride, no doubt due to the 

 fact that the water contains an appreciable percentage of 

 chlorides in solution. A plate which has been insufficiently 

 washed will generally want brightness when dried, and will 

 assume a somewhat brighter appearance and be found to lose 

 slightly in weight if heated to redness. The comparatively 

 simple washing indicated above will generally be found suf- 

 ficient, and the plate may be heated without any sensible loss 

 of weight; but it is well not to continue the electrolysis so 

 long as to put a Yery thick coating of crystals on the plate, 

 as this increases the difficulty in washing and gives no cor- 

 responding increase in accuracy from any other source. 



The plates were dried by heating the end of the plate in a 

 spirit-flame, the greater part of the water having been pre- 

 viously drained off by holding one corner of the plate in 

 contact with a pad of blotting-paper. 



The washing of the copper deposit requires much less care 

 because, if the plate is large enough for the current, it is 

 almost impossible by any ordinary washing to remove copper 

 from the plate. The deposit is solid copper, and may be 

 handled like any other piece of metal without risk of loss. 

 One thing, however, must be attended to, and that is that the 

 plate be not exposed to the air for a longer time than is ab- 

 solutely necessary before the copper sulphate solution has 

 been completely washed off. The reason for this is that a 

 copper plate oxidizes very rapidly when wet with a solution 

 of nearly neutral copper sulphate and exposed to the air. 



The plates should be removed from the solution and at 

 once dipped two or three times into clean water (it need not 

 be distilled), containing two or three drops of sulphuric 

 acid to the litre, and then laid in a tray containing similar 

 water, and washed in the manner above described for the 

 silver plates. The plates may then be lifted out of the acid 

 water, rinsed in clean water containing no acid, and dried, 

 first in a pad of clean white blotting-paper, and afterwards in 

 front of the fire, or, if the plates be small, over a spirit-flame. 



