90 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVI.) 
investing sulphuret of zinc. Hence, therefore, a very sufficient reason, on the che- 
mical theory, why the action should go on. But how can the theory of contact refer 
the current to any contact of the metallic sulphuret, when that sulphuret is, in the 
first place a non-conductor, and, in the next, is dissolved and carried off into the so- 
lution at the moment of its formation? 
1907. Thus all the phenomena with this admirable electrolyte (1880.), whether they 
be those which are related to it as an active (1879.) or as a passive (1825, &c.) body, 
confirm the chemical theory, and oppose that of contact. With tin and cadmium it 
gives an impermeable non-conducting body ; with lead and bismuth it gives an im- 
permeable conducting body; with antimony and silver it produces a permeable non- 
conducting body ; with copper a permeable conducting body ; and with zinc a soluble 
non-conducting body. The chemical action and its resulting current are perfectly 
consistent with all these variations. But try to explain them by the theory of con - 
tact, and, as far as I can perceive, that can only be done by twisting the theory about 
and making it still more tortuous than before (1861. 1865. 1872. 1874. 1889.) ; special 
assumptions being necessary to account for the effects which, under it, become so 
many special cases. 
1908. Solution of protosulphuret of potassium, or bihydros ulphuret of potassa . — I 
used a solution of this kind as the electrolyte in a few cases. The results generally 
were in accordance with those already given, but I did not think it necessary to 
pursue them at length. The solution was made by passing sulphuretted hydrogen 
gas for twenty-fours through a strong solution of pure caustic potassa. 
1909. Iron and platinum with this solution formed a circle in which the iron was 
first negative, then gradually became neutral, and finally acquired a positive state. 
The solution first acted as the yellow sulphuret in reducing the investing oxide (2049.), 
and then, apparently, directly on the iron, dissolving the sulphuret formed. Nickel 
was positive to platinum from the first, and continued so though producing only a 
weak current. When weak chemical action was substituted for metallic contact at 
x, fig. 2 (1831.), a powerful current passed. Copper was highly positive to iron and 
nickel; as also to platinum, gold, and the other metals which were unacted upon by 
the solution. Silver was positive to iron, nickel, and even lead; as well as to pla- 
tinum, gold, &c. Lead is positive to platinum, then the current falls, but does not 
cease. Bismuth is also positive at first, but after a while the current almost entirely 
ceases, as with the yellow sulphuret of potassium (1894.). 
1910. Native gray sulphuret of copper and artificial sulphuret of copper (1899.) 
were positive to platinum and the inactive metals : but yellow copper pyrites, yellow 
iron pyrites, and galena, were inactive with these metals in this solution; as before 
they had been with the solution of yellow or bisulphuret of potassium. This solution, 
as might be expected from its composition, has more of alkaline characters in it than 
the yellow sulphuret of potassium. 
1911. Before concluding this account of results with the sulphuretted solutions, as 
