244 



2nd. If the positive wire is connected with the mercury and the 

 negative wire with the liquid previous to placing both the wires in 

 the electrolyte, films are in nearly all cases instantly produced (but 

 not with strong sulphuric acid) and interfere with further action ; 

 films are also frequently produced by a similar cause upon the end 

 of the mercury nearest to the negative wire when both the wires are 

 in the solution, and in many such cases the mercury creeps in a pecu- 

 liar serpent-like form beneath the film towards the negative wire. 

 3rd. In many instances the metallic globule becomes of a pasty 

 consistence by absorbing substances deposited upon its surface by 

 electrolysis, and the motion declines ; this takes place particularly 

 with mercury in solutions of salts of ammonia, baryta, strontia, 

 magnesia, and lime, but most with those of magnesia and lime ; and 

 it occurs very rapidly if, instead of placing both the polar wires in 

 the electrolyte, the negative wire is immersed in the globule of 

 mercury. It is evident from these facts, that it is essential to the 

 continuance of the movements, that the particles composing the sur- 

 face of the metallic globule should retain a sufficient degree of mo- 

 bility to admit of free motion. 



27 The best method of obtaining a continuous movement is to 

 place a globule of pure mercury in a watch-glass, barely cover it 

 with dilute sulphuric (or nitric) acid, connect it with the negative 

 platinum wire and the liquid with the positive platinum wire of a 

 battery of sufficient power to produce a moderate flow without over- 

 heating the liquid : ten small Smee's batteries are sufficient. By this 

 means I have obtained undiminished motion for upwards of six hours. 



C. Conditions of the direction of the Movements. 



28. In speaking of the direction of the movements, I always 

 mean those of the supernatant liquid, unless otherwise stated, because 

 the true movements of the mercury are generally less easily detected 

 than those of the electrolyte : the movements of the liquid are best 

 observed by means of charcoal or asphaltum (5), and those of the 

 mercury by the aid of a few parallel scratches upon the under sur- 

 face of the watch-glass. 



29. The directions of flow of the metal and liquid are intimately de- 

 pendent upon each other, for in every instance the metal moves in an 

 opposite direction to the electrolyte (see also 22) ; and in those cases 



