1854.] OF THE ROYAL INSTITUTION. 419 
The fact that we very frequently find the double decomposition of a 
salt to be complete, the whole of one of its constituents being pre- 
cipitated, was shown to be easily explained on the principles of 
Berthollet. Thus, for instance, when chromate of potash and 
nitrate of silver are mixed, at the first moment a division will take 
place producing four salts, but one of these —the chromate of 
silver, is thrown down at once as a precipitate, and thus put out of 
the field of action. Another division of the acids with the bases 
must take place, producing of course more of the insoluble chromate, 
and so on till at length the whole of the silver is removed. And 
that this is really what does take place is rendered almost certain by 
the fact that wherever by an interchange of acids and bases a pre- 
cipitate can be produced, that precipitate does form; and, if the 
substance be perfectly insoluble, the whole is thrown down; this 
occurring in opposition to all rules of “ affinity,’’ and to all tables 
that Bergmann, or any other chemist, ever did or could construct. 
The volatility of one of the products acts in the same manner as in- 
solubility, as is exemplified in the decomposition of carbonates by 
any other acid. Crystallization also is but another phase of the same 
phenomenon. An experiment was exhibited in illustration of this. 
Dilute solutions of nitrate of lime, and sulphate of soda, were mixed 
at the ordinary temperature without producing any separation of 
solid matter; but they were so proportioned that upon heating the 
mixture, the crystallization of some sulphate of lime was determined, 
and when once this had commenced, it progressed rapidly; re- 
sembling in that respect the ordinary phenomena of precipitation. 
If in a double decomposition a far larger quantity of a sparingly 
soluble salt be produced at the first moment than the water can dis- 
solve, the crystals will be formed rapidly and will accordingly be 
very small in size; but should there be formed at once only just 
sufficient to determine a separation in the solid form, the crystals 
will grow gradually, and will often attain a large size. This was 
exemplified on the mixture of nitrate of silver with the sulphates of 
copper and of potash respectively. 
It is possible that the law of Berthollet may not be universally 
applicable ; yet the present advanced state of science shows that not 
only is there, as Bergmann insisted, a true chemical affinity, that is— 
a preference of one substance to combine with a certain other sub- 
stance instead of a third,—but, in a great number of instances at 
least, this substance will combine with both according to certain 
proportions, whenever the whole of the affinities can be brought into 
play at the same time. 
(J. HG.) 
