THE ACTION OF CHEMICAL AFFINITY. 
215 
appears to present an exception is that observed by Th. Scherer*, and this requires 
further examination.” The case referred to is this : — if yttria and sesqiiioxide of iron 
be dissolved together in hydrochloric acid, nearly neutralized with ammonia, then 
treated with acetate of ammonia, and afterwards with a few drops of oxalate of am- 
monia, no precipitate falls, but the solution is to a considerable extent decolorized ; 
but the addition of more oxalate of ammonia determines the formation of a white 
curdy precipitate. “It follows from this,” says Scherer, “that hydrochlorate and 
acetate of yttria can be present in a solution simultaneously with ferric oxalate, 
without a precipitate resulting; which indeed is very remarkable, and appears to 
contradict the fundamental laws of chemistry, since oxalic acid gives an insoluble 
precipitate with yttria.” I examined this reaction. The white precipitate which 
was first formed on dropping oxalate of ammonia into the aforesaid mixture, was 
certainly dissipated on further mixing ; yet on repeated trials I always found that 
after standing some hours a small quantity of oxalate of yttria was deposited, while 
the solution still retained the red colour of acetate of iron. However, there is no 
doubt that ferric oxalate, and the yttria salt, did coexist in the same solution ; yet 
in order to prove that no oxalate of yttria was present along with them, it would be 
necessary to show that the salt in question is not dissolved by any of the other sub- 
stances present in the mixture. Now it is by no means insoluble in either hydro- 
chloric or acetic acid, and the conditions of the experiment require the acid to be 
in slight excess. Thus the experiment merely shows that oxalic acid left free to act 
on the ferric and the yttria salts, will combine with the former oxide in very much 
the larger proportion ; a fact which is in perfect consonance with the strong affinity 
between that acid and the sesquioxide of iron, which has been previously remarked 
in this paper. 
It is then a law, without a single known exception, that if AB, CD, EF, &c., by 
any interchange of bases and acids can possibly produce an insoluble substance, that 
insoluble compound does actually make its appearance. This seems to me almost 
conclusive evidence that the interchange always takes place originally to a greater or 
less degree ; for I cannot believe, with one chemist of high repute, that “ when bodies 
are brought into intimate contact, all the forces which exist, not only in themselves, 
but in all their possible compounds, are called into action at the same time,” unless 
indeed it be by these compounds being actually formed. 
The following experiments may illustrate more fully the truth of the explanation 
of complete precipitation which has been given above. ' 
I. Strong solutions of sulphocyanide of potassium and ferric sulphate were mixed. 
The resulting intensely red liquid was divided into two equal parts. The one 
portion was largely diluted with water; and to the other portion a little strong 
alcohol was added, which caused the precipitation of sulphate of potash while ferric 
sulphocyanide was dissolved. The alcohol was poured off, and diluted with water 
* POGGENDORFF, li. 470. 
