8 
ON CHEMISTRY. 
6. The Prussian blue in the other paper set aside, must be weighed 
with its paper, and, so must the portion remaining of the Prussiate 
of potash. Perhaps there are 8 grains of the former, but a part has 
been produced by the test which always contains some iron. If 8 
grains of the dry Prussiate have been employed, allow about one 
fourth of its weight—or two grains, and 6 grains of Prussian blue 
will remain. This latter substance is a prussiate of iron, and in or¬ 
der to detect the exact weight of the oxide of iron in it, fire must be 
be employed to drive off the Prussic acid; but as great difficulty 
must attend this method of analysis, in consequence of the close ad¬ 
hesion of the blue to the paper, I propose that a small quantity of 
hot, caustic alkali—such as soap-lye, or pure potash in solution, be 
dropped into the filter till the blue color pass away, and the sediment 
become of a foxy brown : the prussic acid will be removed', and become 
united to the alkali; and then, 'the paper being washed till no taste 
be present, the substance remaining will be oxide of iron, with per¬ 
haps a grain or two of alumine. The weight of the paper when dried, 
deducted from the gross weight will give that of the oxide of iron. 
7. Test by Sulphuric Acid. The experimenter may either operate 
with the quantity of soil remaining after the abstraction of the iron 
from the parcel tested in the bason, or he may proceed first with 
that in the phial, and apply the prussiate of potash, and solution 
of Carbonate of potash, for the separation of the prussiate of iron 
and Carbouate of lime, as directed above. In either case, he will 
have the same materials to work upon ; but there will, in the latter, 
be a double quantity of the two products, and a like addition to the 
weight of the remaining soil. The 180 grains of each parcel have 
been reduced to about 160 grains, and if we suppose one of these to 
he taken, it will be needful to boil it in a flask or basin, for two or 
three hours, with a diluted sulphuric acid prepared by cautiously ad¬ 
ding 200 grains of the concentrated acid to 800 grains of pure water. 
The substances remaining in the soil are usually the clay or alu¬ 
mine, a further dose of oxide of iron, and a large one of fine siliceous 
earth. The two former are soluble in sulphuric acid, the third is 
not so; consequently, after the due action of the acid the flinty mat¬ 
ter or siliceous earth, is left alone without any other constituent. 
After boiling, the vesssel may remain till its contents become 
cool. Then recourse must be had to the filter for the separation of 
the clear liquid products; and the siliceous earth must be washed as 
before directed. 
The iron is to be precipitated, either by prussiate of potash, or by 
succinate of ammonia; and lastly, the alumine by carbonate of pot- 
