On a General Law of Electrical Discharge. 339 



addition of a small proportion of nitrate of silver, a solution well 

 fitted for use as a preservative agent. The colour of the liquid 

 is a pale rose-red. The nitrate of copper has also been tried for 

 our purpose, but did not give promisuig results, the sensitive- 

 ness of the collodion film being greatly impaired by the highly 

 acid nature of this salt. 



Finally, we have employed with excellent results the nitrate 

 of nickel, which, however, requires some care in its preparation. 

 The method we have found most successful consists in dissolving 

 the metal in the smallest possible quantity of nitric acid, and 

 adding to the solution highly dilute aqueous ammonia, sufficient 

 in amount to precipitate a small portion of the oxide of nickel; 

 this being filtered off", the liquid will have an alkaliue reaction : 

 nitric acid is now added until nearly neutralized, and the last 

 traces of alkalinity removed by acetic acid, a slight excess of 

 which is an advantage. Nitrate of silver should now be intro- 

 duced in the proportion of 2 per cent, of the nickel originally 

 employed. The above mode of proceeding will obviously give 

 rise to the production of a certain quantity of nitrate of ammonia ; 

 this, however, combines to form a double nitrate of nickel and 

 ammonia, a salt possessing deliquescent properties, and appa- 

 rently equally suitable for our purpose. The colour of this agent, 

 bright green, suggests the possibility of employing it with advan- 

 tage in cases where green fohage has to be represented in juxta- 

 position with objects reflecting more active photographic rays. 



Of all the substances known to be applicable to the preserva- 

 tion of collodion plates, we believe that the use of glycerine will 

 give less trouble to those unaccustomed to chemical manipula- 

 tion, and will be generally preferred from the greater certainty 

 of its results. We have nevertheless thought it worth while to 

 record our experience in respect to the other agents severally 

 enumerated, even where, as in the case of fluoride of silver, they 

 have not led to successful results, believing that a statement of 

 the conditions under which we have endeavoured to employ 

 them may save loss of time to future experimentalists in the 

 same direction. 



London, April 14, 1856. 



XLIII. On a General Laiv of Electrical Discharge. 



By Sir W. Snow Harris, F.R.S.* 



[With a Plate.] 



1 . T N a memoir on Electrical Accumulation, presented to the 



JL members of the Plymouth Philosophical Institution so 



long since as the year 1826, and printed in a volume of their 



♦ Communicated by the Author. 



