104 
ON THE DOUBLE SALTS OF IRON. 
in such proportion as to keep the bitartrate slightly in ex- 
cess, supersaturating at the end with ammonia, a salt will 
be obtained perfectly but slowly soluble in cold water, sub- 
stantially consisting of tartaric acid, iron, and potash, and 
belonging to the superseries of double salts. The admix- 
ture of ammonia is advised, first, because it is impossible to 
separate any excess of potash, should there be any; and, 
secondly, because it is difficult to effect complete combination 
without such excess. 
In pursuing this investigation, the present nomenclature 
of the Pharmacopoeia seemed incapable of comprehending 
the double series which opened itself to view; and on 
searching for a better, I soon found myself on ground which 
was occupied by contending parties. The unsettled point 
consisted in this: Is the iron, or the third constituent, to be- 
come the adjective adjunct to the acid ? In other words, 
is a compound of tartaric acid, oxide of iron, and potash, to 
be called potassio-tartrate of iron or ferro-tartrate of potash? 
Many, whose names command the greatest respect, con- 
tinue to make the third constituent express the quality of 
the acid ; and these have custom for their stay. On the 
other hand, it is contended that this nomenclature conveys 
erroneous impressions of the constitution of the salt ; and 
the argument is thus summed up by Dr. Pereira in his 
Elements of Materia Medica: " The ferro-tartrate of pot- 
ash is to be regarded as a double salt, in which tartrate of 
iron is the acid, or electro-negative ingredient, and tartrate 
of potash the electro-positive, or basic, constituent. On this 
view we comprehend why ferro-cyanide of potassium and 
the alkalies refuse to act on it in the way they do on the 
ordinary ferruginous salts until an acid be added." But 
the author, in adopting a nomenclature, leaves the double 
protos&lts unprovided for. If, then, the electric condition 
of these oxides is such that we are obliged to regard them 
in the light of acids, rather than bases, it seems consistent 
that, with Berzelius, we should apply the usual termination 
