398 Mr. T. Gray on the Electrolysis 



this case it is not necessary, after the first trial at least, to 

 adjust the current with the plates in position, as the resistance 

 of the cells can be nearly enough allowed for. In these cases 

 the plates were simply polished either with silver sand and 

 water or with clean sand-paper, then washed in clean water 

 to remove the sand, placed in water slightly acidulated with 

 sulphuric acid for a short time so as to remove any trace of 

 oxide, then rinsed in clean water, and dried first in clean 

 blotting-paper and afterwards in front of the fire or over a 

 spirit-flame. If the plate be thickly oxidized on the surface 

 but otherwise clean, as is often the case with new sheet 

 copper, the quickest mode of cleaning is to wet the plates and 

 then place them for a few seconds in strong nitric acid, rinsing 

 them immediately afterwards in clean water, and putting 

 them into water containing a few drops of sulphuric acid to 

 prevent oxidation. The plates may then be lifted one by one 

 out of the acidulated water, rinsed in clean water, and dried 

 in the manner above described. 



The method of drying here recommended is important, as 

 the blotting-pad removes almost at once nearly the whole of 

 the water, and hence the drying can be completed in a few 

 seconds, either in front of a fire or over a spirit-flame, with- 

 out the slightest oxidation taking place. A plate may be 

 washed and dried in this manner over and over again without 

 producing as much as the tenth of a milligramme of difference 

 in its weight. 



Washing the Deposit. — The operation of washing the de- 

 posit at the conclusion of the electrolysis is one on the im- 

 portance of care in which Lord Rayleigh has laid great stress. 

 The method adopted for silver in these experiments was to 

 lift the bar b (fig. 1) with the plates attached, and either to 

 dip them several times in clean distilled water contained in 

 a clean glass vessel before removing them from the clips, or 

 to remove them as quickly as possible from the clips, and dip 

 them several times one by one into the water; in either case 

 observing carefully whether any small crystals were removed 

 in the operation. If there is danger of loss of silver from the 

 anodes, it is better to remove the plates from the clips before 

 they are dipped in the water. After this preliminary rinsing 

 to wash off the greater part of the silver nitrate solution, the 

 plates were laid in the bottom of a shallow glass tray con- 

 taining distilled water, and the water made to flow backwards 

 and forwards over them for a minute or two by raising and 

 lowering one edge of the tray. The plates were then re- 

 moved and placed in another similar tray containing clean, 

 but not necessarily distilled, water, washed in a similar 



