102 7. S. Hunt on Solution and the Chemical Process. 
Solution being then identification, the discussion as to whether 
metallic chiorids are changed into hydrochlorates when dissolved 
in water, is meaningless. Such a solution is a unity, in which 
we can no more assert the existence of the chlorid or of water, 
than of chlorine, hydrochloric acid, or a metallic oxyd, although 
these and many others are conceivable results of its differentia- 
tion. If the solution be one of chlorid of potassium, evapora- 
tion resolves it into water and the chlorid, but if chlorid of alu- 
minum, it is decomposed by boiling into water, hydrochloric acid, 
and alumina, or in the case of the magnesian salt, into hydro- 
chlorie acid and an oxychlorid. 
The precipitation of the sulphates of cerium, lanthanum and . 
lime from their solutions by heat, and of most other salts by cold, 
is chemical decomposition or differentiation, Dilution may also 
effect decomposition in solutions; we have already said that the 
combination of terchlorid of arsenic AsCle, with 36 is sta- 
ble at ordinary temperatures, but a further addition of water 
canses the solution to divide into aqueous hydrochloric ngs and 
crystalline oxyd of arsenic. The precipitation of chlorid of an- 
timony, and many salts of bismuth and mercury by wa ae is an 
analogous process. ‘lhis decomposition of the solution of chlorid 
of arsenic is an example of what is called double elective affinity, 
(attractio electiva duplex,) and is generally explained by saying 
that the attraction of arsenic for oxygen, and that of chlorine for 
hydrogen, enable the chlorid and water to decompose each other. 
But these elemental species do not exist in the solution, although 
they are possible results of its decomposition, and to explain the 
process in this manner is to aseribe it to the afliuities of yet un- 
formed speries 
have elsewhere asserted that double decomposition always 
involves union followed by division,* although we cannot in every 
case arrest the process at the first stage. "Under some changed 
conditions of temperature and pressure, the decomposition may 
be the counterpart of the previous union, and thus ih aa the 
original species, as in the case of mercuric oxyd, which i 
composed into mercury and oxygen ata temperature a little oe 
that at which it was formed. When the division takes place in 
a sense different from the union, giving rise to new species, we 
have double decomposition. In the case of chlorid of arsenic, 
the aqueous solution exhibits the first stage of the process. A 
similar condition of austable union is observed in many other ite 
stances; thus binoxyd of manganese gives with cold hydro- 
chloric acid, a brown solution, but the combination is by a gentle 
heat resolved into chlorine gas, aud a rose-red solution of proto- 
chlorid of manganese. So a mixture of equivalent parts of 
* Consderations on the Theory of Chemical Changes, ete., cited above, 
