Mr. Joule on the Electrical Origin of Chemical Heat. 207 
of sulphuric acid. Consequently there were plenty of atoms of 
water either uncombined, or only slightly attached to the acid, 
prepared to give up their elements to the current with little or 
no additional resistance in consequence of its presence. 
By inspecting the table it will be seen that these corrected 
theoretical results agree very well with the experiments of 
Dulong and myself. They accord accurately in the case of 
zinc. Iron gives results which are not equally satisfactory. 
But we must remember that it is converted by combustion 
into the magnetic oxide, and that a correction ought therefore 
to be applied on account of heat evolved by the union of pro- 
toxide with oxygen, which it is very difficult to prevent en- 
tirely. Potassium gives theoretical and experimental results 
as nearly alike as can, I think, be expected, considering the 
complicated process* by which the former were obtained, and 
the practical difficulty of the latter. In the case of hydrogen 
we might have anticipated that theory would exceed experi- 
ment; for the resistance to electrolysis of water appears gene- 
rally greater than it really is, on account of the peculiar state 
which the platinum evolving hydrogen is apt to assume, which 
has, of course, the effect of increasing the theoretical value. 
Besides the corrections to the theoretical results which I 
have supplied, I thought that there might be a slight one 
needed on account of light which is evolved in such abundance 
in some instances of combustion. It was of importance to as- 
certain whether in the evolution of light an equivalent of heat 
was absorbed. With this view I have made an extensive 
series of experiments with the voltaic apparatus, comparing 
the heat evolved when no light was exhibited, with that evolved 
when the conducting wire was ignited to whiteness. ‘The mean 
of twelve experiments showed that the heat evolved by a cer- 
tain quantity of wire immersed in water is, for a given quan- 
tity of current and a given length of time, 24°75; and the 
mean of sixteen experiments, in which a platinum wire inclosed 
in a glass tube surrounded with water was ignited so as to 
give out a quantity of light equal to that arising from a com- 
mon tallow candle, gave 24°°4 as the quantity of heat due in 
this latter case to similar circumstances of resistance and quan- 
tity of current. These experiments seem to indicate that heat 
is lost when light is evolved, but in so slight a degree that my 
experiments on the heat of combustion need not be corrected 
for it. Dulong’s experiments were performed in a box of cop- 
per, which being opake would entirely obviate this source of 
error. 
I conceive that the correctness of the idea, entertained, I 
* Phil. Mag. S. 3, vol. xx. p. 109 (49). 
