as connected with the Theory of Substitutions. 363 
the resulting effect is not a temporary thing, but one which lasts 
for a considerable period of time, as appears to be proved in the 
Philosophical Magazine, (July, 1844,) we can give a very plain 
and simple account of the decomposition of water by this gase- 
ous substance under the influence of sunshine. 
pon the same principle that a mixture of chlorine and hydro- 
gen may be kept in the dark without union for a long time, so 
may a solution of chlorine in water be preserved. The chlorine 
is in an inactive state. 
But, if any thing is done to make the chlorine take on its other 
form and pass to the active condition ; if it be, for example, set in 
the sunshine, its affinity for hydrogen is exhibited, and decompo- 
sition is the result. 
The qualities thus communicated to the chlorine not being of 
a transient kind, but remaining for a length of time, we see how 
itis that after an exposure to the sun decomposition is subsequently 
carried forward in the dark. 
The indisposition of chlorine to unite with carbon, which has 
been regarded as a singular quality, is not more remarkable than 
its indisposition to unite with hydrogen in the dark. 
If the power which chlorine assumes of uniting with hydrogen 
and carbon depends on a change in its electrical relations,—a 
passage from the passive to the active state,—we might expect 
that those various causes which in the case of other elementary 
bodies bring about analogous changes, and throw them from one 
allotropic condition to another, would here also exercise a percept- 
ible action. Among such causes we may enumerate the action. 
of a high temperature, and the contact or presence of other bodies. 
It may be remarked in the instances to which Berzelius has 
referred, that exposure to a high temperature is one of the most 
frequent causes of allotropic change. In the case of chlorine the 
temark holds good, for, as is well known, when a mixture of 
chlorine and hydrogen is passed through a red-hot tube, chloro- 
hydric acid forms with rapidity. ‘The high temperature, there- 
fore, impresses upon chlorine the same tendency to unite with 
ydrogen which is communicated by the solar rays. 
But the contact of other bodies frequently determines in a given 
Substance an allotropic change. ‘Thus, when a piece of iron is 
Placed in nitric acid in contact with platina, the iron becomes 
less electro-positive, or what is the same thing, more electro-neg~ 
