218 Prof. Davy on some further applications of 
bleaching powder from a burette till all the ferrocyanide was con- 
verted into ferridcyanide) 1s, I find, not a good manner of employ- 
ing the ferrocyanide in this estimation, and, like the other 
methods, will lead to aloss of chlorine; for when the solution of 
chloride of lime is added to the acidified ferrocyanide, a portion 
of the chlorine is. separated, especially if the bleaching liquor be 
added too quickly, or is not greatly diluted. But the way I pro- 
pose of using the ferrocyanide of potassium in this important 
valuation, is to mix together a certain quantity of a standard solu- 
tion of ferrocyanide with a given amount of a graduated solution 
of the chloride of lime, using more of the former salt than the 
latter can convert into ferridcyanide ; then adding hydrochloric 
acid to dissolve the precipitate formed and render the mixture 
strongly acid, and finally ascertain, by means of a standard solu- 
tion of bichromate of potash, how much of the ferrocyanide_re- 
mained unconverted into the ferridcyanide by the action of the 
chlorine of the chloride of lime,—which is effected by adding 
slowly from a graduated burette the standard solution of bichro- 
mate till a minute drop taken from the well-stirred mixture by 
means of a glass rod, ceases to give, with a small drop of a very 
dilute solution of perchloride of iron placed on a white plate, a 
blue or greenish colour, but produces instead a yellowish brown*. 
When this latter effect is observed, it indicates that all the ferro- 
cyanide has been converted into ferridcyanide; and as 147-59 
(one equivalent) of bichromate of potash is capable of converting 
1267°32 (six equivalents) of crystallized ferrocyanide of potassium 
into ferridcyanide, and as 422°44 (two equivalents) of the ferro- 
cyanide are converted into the same substance by 35°5 (one 
equivalent) of chlorine, as is seen by the formule already given, 
knowing the amount of chloride of lime employed, we have all 
the data necessary to calculate the per-centage of chlorine. 
Having made two standard solutions, the first contaiming 
21:122 grammes of ferrocyanide of potassium in a litre of the 
solution, and the second 14°759 grammes of bichromate of pot- 
ash in the same quantity of solution (weights which are to each 
other as their atomic equivalents), | made several estimations of 
chloride of lime with them, adopting the method I have just 
described, and found that it gave the most consistent results, and 
which agreed very closely with those obtained by Gay-Lussac’s 
and Otto’s methods when the latter were performed with the great- 
est care,—the only difference being that the results obtained by 
* The yellowish-brown coloration which is at first produced when 
enough of the bichromate has been added, quickly changes to a greenish 
colour by some secondary reactions which take place when the persalt of 
iron is left in contact with the mixture. but this does not interfere with 
the test ; for it is the first effect which is produced which indicates the com- 
pletion of the reaction, and not the after changes which may result. 
