the Ferrocyanide of Potassium in Chemical Analysis. 217 
useful determinations, the first, and one of the most important, 
of which is the ascertaining the amount of available chlorine in the 
chloride of lime or bleaching powder, which is a matter of much 
importance in many of the chemical arts, but particularly in 
bleaching ; for not only does the commercial value of this sub- 
stance depend on the quantity of available chlorine that it con- 
tains, which is subject to great variation from exposure to the 
air and other causes, but likewise it is of the greatest importance 
that the bleacher should readily be able to determine from time 
to time the strength of the bleaching liquor which he employs: 
for if it be too strong, he knows that the fabric which he bleaches 
will be injured; and if too weak, it will not be sufficiently bleached, 
and the process must be repeated, which mcurs much additional 
expenditure of time. 
Various methods have from time to time been proposed for the 
determination of the value of chloride of lime; but the greater 
number of them, from the trouble required to make the test-so- 
lutions, and their not keeping when made, as well as the skill re- 
quired in their use, render them inapplicable for general purposes. 
I shall therefore merely refer to the two methods which are 
chiefly used at present to determine the value of this important 
substance. The first is Gay-Lussac’s, in which the amount of 
chlorine is ascertained by seeing how much chloride of lime is 
necessary to convert a given quantity of arsenious into arsenic 
acid ; the second is Otto’s, in which protosulphate of iron is sub- 
stituted for arsenious acid, and the determination of chlorine is 
made by seeing how much of the bleaching powder is required 
to change a given weight of the protosulphate of iron into a per- 
salt of that metal: these processes are so well known that I need 
not describe them. 
In both these methods I find that more or less chlorine is 
always lost, which, however, may be reduced toa minute quantity 
by very carefully adding the solution of chloride of lime either 
to that of arsenious acid, or of protosalt of iron ; but in ordinary 
hands they (especially the latter process) will yield results in 
which too small a proportion of chlorine will be indicated, from 
the loss of that substance which will invariably take place. 
The ferrocyanide of potassium answers admirably for the 
estimation of available chlorine in the chloride of lime, when used 
in the manner I shall presently explain, and according to my 
experiments will give in ordinary hands far more accurate results 
than either Gay-Lussac’s or Otto’s method. I am aware, in- 
deed, that this salt was proposed by Mr. Mercer some years ago 
for this purpose; but the way which he recommended it to be 
used (which consisted in dissolving a certain weight of the ferro- 
cyanide in water, acidifying it, and then adding the solution of 
