540 PoRRETT on the nature of the Salts 
other substance which the ferruretted chyazate is known to ' 
precipitate, then it is necessary to separate such substance by 
the usual means, before the application of this test for iron. 
With the knowledge of these facts, therefore, it is easy 
always to obtain correct results, which will best appear by 
assuming a case particularly unfavourable for the purpose, 
and shewing how the difficulties of that case may be ob- 
viated. 
' Let it therefore be supposed that the analyst has a solution 
having a considerable excess of nitromuriatic acid and con- 
taining oxide of iron, barytes, alumine, magnesia, and oxide 
of copper, and that he wishes to precipitate the iron by the 
ferruretted chyazate of potash. If he first throw down the 
barytes by a sulphate, then the other earths and oxides by 
ammonia, adding an excess of ammonia to redissolve the 
oxide of copper, if he then separate the alumine by diges- 
tion in caustic potash, and redissolve the oxide of iron and 
magnesia in an acid, which for the sake of increasing the dif- 
ficulty may again be supposed to be the nitromuriatic ; I say 
then, that by neutralizing any very great excess of acid, and 
adding some acetite of potash in order that the unavoidable 
excess of acid may be the acetic instead of the nitromuriatic; 
then applying the ferruretted chyazate of potash, taking care 
that there be but little, if any, excess of it added ; he may 
safely boil the liquid and readily collect by the filter, or by 
subsidence, the ferruretted chyazate of per-oxide of iron, 
which when properly' washed, dried, and weighed, will indi- 
cate very accurately the quantity of iron that was in the solu- 
tion, by reckoning that quantity as 34.235 of per-oxide, for 
every 100 parts of Prussian blue obtained. 
