440 M. BECQUEREL ON CHEMICAL DECOMPOSITION AND 
in the constituent parts of which we wish to produce changes; and the 
third, with water rendered a slight conductor of electricity by the ad- 
dition of an acid or common salt capable of acting chemically on the 
metal which is to be immersed in it. A communicates with A! by 
means of a bent tube, abc, filled with potter’s clay moistened with a 
saline solution, the nature of which depends on the effect intended 
to be produced in A; A!’ and A’ communicate with each other 
through the medium of a plate (a'b'c') of platina or gold; and A 
and A" communicate by means of a voltaic pair (CM Z), composed 
of two plates (MC and MZ) of copper and zinc; in fine, a safety-tube 
(tt) is placed in the vessel A’, in order to indicate the internal pres- 
sures resulting from the disengagements of gas. According to this 
arrangement, the extremity a! of the plate of platina is the positive pole 
of a voltaic apparatus whose action is slow and continuous. When the 
liquid contained in A! is a good conductor, the intensity of the electric 
forces is sufficient to decompose the sulphate of copper in A. From 
that instant the oxygen proceeds towards a’, as well as the sulphuric 
acid, which, in passing into the tube (abc) expels those acids which 
have a less affinity than it has itself towards the bases. These acids 
and the oxygen pass into the liquid A’, where their slow reactions de- 
termine the relative changes in the bodies which they find there. This 
apparatus allows us to operate on a greater scale, and to avoid the re- 
action of the oxide in the same manner as it was avoided in the appa- 
ratus previously described. 
We are often compelled to place a fourth jar between A and A’, into 
which there is put so much of a saline solution, to be decomposed by 
the sulphuric acid, that the effects produced in the liquid A’ may not 
be interrupted when all the liquid of the clay has been decomposed. 
Thus, when we wish to bring a gas or an acid in its nascent state into 
the liquid of the vessel A’, we have only to introduce into the clay 
a solution which, by its reaction on the sulphuric acid proceeding 
from the decomposition of the sulphate of copper, may let the gas or the 
acid escape. If, on the contrary, it should be found necessary to con- 
duct into the same liquid either hydrogen or an electro-positive gas, 
