1851.] OF THE ROYAL INSTITUTION. 93 
The exchange of analogous particles actually constitutes double 
decomposition ; and its occurrence in alternately opposite directions 
causes the two substances used to alternate with the two other com- 
pounds formed by the exchange of their bases ; so that in such a mix- 
ture, four substances are constantly to be found, the quantity of each 
substance corresponding to the average number of atoms which, in 
each moment of time, are in that state of combination. 
Now it is clear that if an equal number of atoms of a hydrogen- 
salt, and of an iron-salt, reacting on one another, form a certain 
amount of the products of their double decomposition, a greater 
number of those products will be formed by doubling the quantity of 
the hydrogen-salt; for, the facility of interchange of iron with 
hydrogen remaining the same, the atoms of the iron-salt will then 
come more frequently in contact with those of the hydrogen-compound. 
Thus, on mixing a solution of sesquichloride of iron with sulpho- 
cyanide of hydrogen, a deep red colour gave evidence of the inter- 
change of iron and hydrogen, forming sulphocyanide of iron and 
hydrochloric acid: but this exchange was not an operation effected 
once for all on the decomposing substances just coming in contact, 
but is constantly going on in the mixture; andthe quantity of the 
products of this interchange remains constant, because a similar 
double decomposition, equal in absolute number of atoms per unit of 
time, is constantly going on between these products, reproducing the 
original compounds. Inevidence of this, the counterpart of the 
decomposition of sesquichloride of iron by hydrosulphocyanic acid 
was shown in the expulsion of this last acid by hydrochloric, 
proved by the gradual diminution of the red colour on adding 
hydrochloric acid. 
It is well known that caustic soda expels ammonia from its salts. 
But ammonia also expels soda: for it was shown, that a mixture of 
ammonia and hydrochlorate of soda dissolves less chloride of silver 
than the same quantity of ammonia alone; and consequently, that a 
saturated solution of chloride of silver in aqueous ammonia is pre- 
cipitated by dissolving chloride of sodium in it. The same occurs 
with the ammonio-sulphate of copper. 
In conclusion, the lecturer referred to the question of the relative 
velocity of transfer of analogous atoms in opposite directions, which 
necessarily determines the proportion of the elements of two salts, 
contained in the form of their products of double decomposition, 
on these salts being mixed. On the mixture of equivalent pro- 
portions of a couple of salts in aqueous solution, a certain amount of 
decomposition ensues, forming two other salts, and the chemical force 
may be considered proportional to the quantity of one couple compared 
to that of the other. Now as the proportion is only kept up by the 
number of exchanges in the one direction being ansoLuTELY the same 
in each moment of time as those in the opposite direction, it is 
clear that the relative velocity of interchange must be greatest be- 
tween the elements of that couple of which the quantity is least ; and 
