100 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
to the cold, 60° or more. Iron and diluted nitric acid gave even a still more striking 
result. 
I must now enumerate merely, not that the cases to be mentioned are less decided 
than those already given, but to economize time. 
1947- Dilute solution of yellow sulphuret of potassium, consisting of one volume of 
the strong solution (1812.), and eighteen volumes of water. — Iron, silver, and copper, 
with this solution, gave good results. The hot metal was positive to the cold. 
1948. Dilute solution of caustic potassa (1932.). — Iron, copper, tin, zinc, and cad- 
mium gave striking results in this electrolyte. The hot metal was always positive 
to the cold. Lead produced the same effect, but there was a momentary jerk at the 
galvanometer at the instant of immersion, as if the hot lead was negative at that 
moment. In the case of iron it was necessary to continue the application of heat, and 
then the formation of oxide at it could easily be observed : the alkali gradually became 
turbid, for the protoxide first formed was dissolved, and becoming peroxide by degrees, 
was deposited, and rendered the liquid dull and yellow. 
1949. Dilute sulphuric acid (1935.). — Iron, tin, lead, and zinc, in this electrolyte, 
showed the power of heat to produce a current by exalting the chemical affinity, for 
the hot side was in each case positive. 
1950. Dilute nitric acid is remarkable for presenting only one case of a metal hot 
and cold exhibiting a striking difference, and that metal is iron. With silver, copper, 
and zinc, the hot side is at the first moment positive to the cold, but only in the 
smallest degree. 
1951. Strong nitric acid. — Hot iron is positive to cold. Both in the hot and cold 
acid the iron is in its peculiar state (1844. 2001.). 
1952. Dilute muriatic acid: 1 volume strong muriatic acid , and 29 volumes water . — 
This acid was as remarkable for the number of cases it supplied as the dilute nitric 
acid was for the contrary (1950.). Iron, copper, tin, lead, zinc, and cadmium gave 
active circles with it, the hot metal being positive to the cold ; all the results were 
very striking in the strength and permanency of the electric current produced. 
1953. Several cases occur in which the hot metal becomes negative instead of posi- 
tive, as above ; and the principal cause of such an effect I have already adverted 
to (1918.). Thus with the solution of the, sulphuret of potassium and zinc, on the 
first immersion of the wires into the hot and cold solution there was a pause, i. e. 
the galvanometer needle did not move at once, as in the former cases ; afterwards a 
current gradually came into existence, rising in strength until the needle was deflected 
70° or 80°, the hot metal being negative through the electrolyte to the cold metal. 
Cadmium in the same solution gave also the first pause and then a current, the hot 
metal being negative ; but the effect was very small. Lead, hot, was negative, pro- 
