348 Dr. Woods on the Time required 
tive force of the pairs; this force is therefore synonymous with 
rapidity of current. They also represent the difference between 
the heat generated at the positive end of the battery by the for- 
mation of sulphate of zine, and that absorbed at the negative end 
by the decomposition. The latter statement may be proved by 
subtracting the amount of heat absorbed by decomposition in 
each particular case from that generated at the positive end. 
Regnault: has proved it in a paper, published, I think, in the 
August number of the French ‘ Annals of Chemistry’ for 1855 ; 
a paper I unfortunately did not see until I had made many ex- 
periments myself for the same purpose. 
In all these pairs the same quantity of electricity is developed 
by the consumption of an equivalent of zine, but the time it 
requires to make the circuit depends on the time the compound 
at the negative end takes to be decomposed ; and this amount of 
time depends, again, on the quantity of heat which such decom- 
position absorbs. Now this proposition leads us to the interest- 
ing fact, that all compounds require the same time to absorb the 
same quantity of heat in decomposing into their elements. For 
instance, the decomposition of water absorbs 80* units of heat, 
that of nitric acid 16*, that of sulphate of copper 62*, of nitrate of 
silver 30*, &c.; and the time they require to do so is exactly 
proportional to these amounts. 
I may here observe, parenthetically, that from the above we 
can understand the difference of facility with which compounds 
can be decomposed. All compounds absorbing much heat m 
decomposing, such as the alkalies, &c., require a comparatively 
long time to separate into their elements; whilst bodies which 
absorb very little heat, decompose at once and from the most 
trifling causes; for instance, all explosive compounds, such as 
the fulminates, hypochlorous acid, &c.: these, not requiring time 
to absorb heat when decomposing, allow the affinities which form 
the elements into new compounds to satisfy themselves most 
easily and rapidly. 
But the ¢ime required by compound bodies to absorb heat in 
their decomposition is only equal under similar circumstances ; 
for instance, when zinc is the positive pole of a galvanic couple, 
an equivalent of all compounds at the negative decomposes with 
a rapidity proportional to the quantity of heat they absorb, 
Thus nitric acid, which absorbs only about one-third of the heat 
that water does, decomposes three times as quickly, and hence 
an equivalent of zine is dissolved in one-third of the time in a 
Grove’s pair that it is in a Daniell’s pair; and an equivalent of 
electricity passes with proportionate quickness. But if potassium 
be made the positive metal in one of the pairs, the rapidity of 
* Proceedings of the Royal Society, January 1857. 
