by Compounds for Decomposition. 349 
current increases in proportion to the increased affinity of that 
metal for oxygen. For it seems a curious fact, that some elements, 
when combining, evolve heat more or less quickly than others ; 
but in separating, the heat necessary for decomposition is ab- 
sorbed equally ; and the greater the quantity of heat an element 
generates in combining, the more rapidly it evolves it. Hence, 
as was mentioned before, the rapidity of the electric current de- 
pends on the difference of amount of heat evolved at one end and 
absorbed at the other of the circuit. If a very rapid current is 
required, a metal which produces the greatest amount of heat is 
made the positive pole, and a liquid which absorbs the least 
quantity of heat is placed round the negative one; both ends 
thus entering into the development of the current. 
Faraday says that the current always sets out from the metal 
most acted on; but this does not always hold good. It is quite 
possible to have a metal easily acted on by an electrolyte which 
is a good conductor, and also to have a good negative pole, and 
yet no current of electricity, because the decomposition may not 
be such as to allow the current to pass. For instance, copper 
placed in nitric acid in a porous cell, and platina in sulphuric 
acid around it, gives no current of electricity, although the cop- 
per is quickly acted on, because the decomposition of the water 
at the negative end requires more time to be accomplished than 
the combination at the positive end. If the position of the 
metals be reversed, a strong current passes. If nitric and sul- 
phuric acid be separated by a porous diaphragm, and a plate of 
copper be placed in each, the copper in the sulphuric acid acts as 
the positive metal, although sulphuric acid attacks the metal much 
less strongly than the nitric acid. Whenever, therefore, we cal- 
culate the galvanic effect of a pair, one end of the circuit is of 
just as much consequence as the other, whether we regard the | 
amount or direction. The electricity is developed at the positive 
end, but it must pass through the negative end; and on the 
latter depends not only the rapidity with which it does so, but 
on it depends also whether it passes at all or not; as, if a fluid 
is used which takes more time to be decomposed than the ele- 
ments at the positive end require to combine, no current can be 
produced. 
One of the objects of this paper is to direct the attention of 
scientific men to the importance of the study of the heat of che- 
mical combinations. If the heat produced by the combinations 
of all the elements were known, we should have a key to almost 
all the pheenomena connected with the principles of chemistry. 
The reactions of bodies depend on, or are inseparably connected 
with, the amount of heat their combinations produce: in every 
instance the result of contact of bodies with respect to affinity 
