214 
DR. GLADSTONE ON CIRCUMSTANCES MODIFYING 
TESTIMONY FROM OTHER CHEMICAL PHENOMENA. 
There are many chemical phenomena, beside those connected with colour, which 
bear testimony respecting the question whether two salts in solution resolve them- 
selves into four. 
The testimony of precipitation . — The idea that when double decomposition occurs, 
the acids and bases make a perfect exchange, arose doubtless from what is constantly 
observed when a precipitate ensues. In that case A combines wholly with D, and 
C with B. Yet this will be the inevitable result under the one theory as well as 
under the other. A mixture of single equivalents of nitrate of baryta and sulphate 
of potash may be taken as an illustration. Here, as has frequently been shown, if 
Bekthollet’s views be correct, at the first moment of mixing a portion of the baryta 
combines with sulphuric acid, but that compound being insoluble is instantly put 
out of the field of action, and the resulting mixture really consists of nitrate of baryta, 
nitrate of potash, and sulphate of potash, which of course gives rise to a redistribu- 
tion of the bases and acids, and a further production of insoluble sulphate of baryta, 
and so on, till the amount of nitrate of baryta retnaining is infinitesimally small ; 
while at the same time the whole of the potash must necessarily combine with the 
whole of the nitric acid. It is scarcely necessary to observe, that this division and 
precipitation will take place continuously until complete ; and that it may be so 
rapid as to elude our notice*. The fact then that precipitation when it occurs is 
complete, decides nothing as to the relative merits of the two theories of elective 
affinity. Yet there is an important difference to be noted. On Bergman’s suppo- 
sition, it can hardly be imagined but that cases will sometimes occur, where A has so 
strong an affinity for B, or C so powerful an attraction for D, that on mixing AB 
and CD, no interchange will take place, although AD may be an insoluble body. 
On Berthollet’s supposition, the insoluble compound will always be wholly precipi- 
tated whenever by the interchange of acids and bases such a compound can be 
formed, even though it be against the preponderating direction of the affinities. Now 
this can be put at once to the test of experience : and what is the testimony of the 
thousands of double decompositions which chemists are in the habit of meeting with r 
Graham'I' says, “ It is a general law to which there is no exception, that two soluble 
salts cannot be mixed without the occurrence of decomposition, if one of the pro- 
ducts that may be formed is an insoluble salt.” Givjelin:|;, even when arguing 
against Berthollet’s views, admits the sanie fact, adding, “the only case which 
* Yet it is easily conceivable that when the affinity for each other of the two substances that produce the 
insoluble compound is very weak, the action may last some time, and become evident to our senses. Is not 
this actually the case when sulphate of lime in solution is added to nitrate of strontia, or carbonate of soda to 
chloride of calcium, or an alkaline carbonate to tartrate of yttria, or oxalate of ammonia to sulphate of mag- 
nesia, &c..^ 
t Elements of Chemistry. 
X Handbook of Chemistry. 
