SOURCE OF VOLTAIC ELECTRICITY. — PRECAUTIONS. 
95 
tin and cadmium in dilute sulphuric acid gave a strong current, the cadmium being- 
positive, and the needle was deflected 80°. When left, the force of the current fell 
to 35°. If the cadmium were then moved it produced very little alteration ; but if 
the tin were moved it produced a great change, not showing, as before, an increase 
of its force, but the reverse, for it became more negative, and the current force rose 
up again to 80°*. The precaution adopted to avoid the interference of these actions, 
was not only to observe the first effect of the introduced wires, but to keep them 
moving from the moment of the introduction. 
1920. The above effect was another reason for heating the acids, &c. (1918.) before 
the wires were immersed; for in the experiment just described, if the cadmium side 
were heated to boiling, the moment the fluid was agitated on the tin side by the 
boiling on the cadmium side, there was more effect by far produced by the motion 
than the heat : for the heat at the cadmium alone did little or nothing, but the jump- 
ing of the acid over the tin made a difference in the current of 20° or 30°. 
1921. Effect of air. — Two platinum wires were put into cold strong solution of sul- 
phuret of potassium (1812.), fig. 7 ; and the galvanometer was soon at 0°. On heating 
and boiling the fluid on the side A (1915.) the platinum in it became negative ; cool- 
ing that side, by pouring a little water over it from a jug, and heating the side B, the 
platinum there in turn became negative ; and, though the action was irregular, the 
same general result occurred however the temperatures of the parts were altered. 
This was not due to the chemical effect of the electrolyte on the heated platinum. 
Nor do I believe it was a true thermo current (1933.) ; but if it were the latter, then 
the heated platinum was negative through the electrolyte to the cold platinum. I 
believe it was altogether the increased effect of the air upon the electrolyte at the heated 
side ; and it is evident that the application of the heat, by causing currents in the fluid 
and also in the air, facilitates their mutual action at that place. It has been already 
shown, that lifting up a platinum wire in this solution, so as to expose it for a moment 
to the air (1827.), renders it negative when reimmersed, an effect which is in perfect 
accordance with the assumed action of the heated air and fluid in the present case. 
The interference of this effect is obviated by raising the temperature of the electrolyte 
quietly before the wires are immersed (1918.), and observing only the first effect. 
1922. Effect of heat. — In certain cases where two different metals are used, there 
is a very remarkable effect produced on heating the negative metal. This will require 
* Tin has some remarkable actions in this respect. If two tins be immersed in succession into dilute nitric 
acid, the one last in is positive to the other at the moment : if, both being in, one be moved, that is for the 
time positive to the other. But if dilute sulphuric acid be employed, the last tin is always negative : if one be 
taken out, cleaned, and reimmersed, it is negative : if, both being in and neutral, one be moved, it becomes 
negative to the other. The effects with muriatic acid are the same in kind as those with sulphuric acid, but not 
so strong. This effect perhaps depends upon the compound of tin first produced in the sulphuric and muri- 
atic acids tending to acquire some other and more advanced state, either in relation to the oxygen, chlorine 
or acid concerned, and so adding a force to that which at the first moment, when only metallic tin and acid are 
present, tends to determine a current. 
