VALUATION OP BLEACHING POWDER. 279 
we will suppose to be 70 measures, contains always 10 
grains of chlorine, it becomes a simple matter to calculate 
the per centage of chlorine in the specimen." 
This mode of operating offers some serious inconve- 
niences, and is productive of error. 
The. first is, that it is very difficult to have pure sulphate 
of protoxide of iron, for it oxidizes with great facility 5 and 
unless in the hands of a chemist, or one well acquainted 
with its nature, great errors may arise in employing it as a 
standard. 
A second objection arises from the necessity of having 
the salt in a fit state of dryness, which is not easy to be 
ascertained by a commercial person ; and lastly, the disad 
vantage of not being able to judge with promptness the 
exact point when the oxidation is complete. 
This is why I would employ Gay Lussac's method, whose 
standard liquors can be kept a great length of time without 
undergoing any change ; and further, because we are 
enabled readily to perceive the instant of saturation, either 
by natural or artificial light. 
This system is founded on the readiness "with which 
arsenious acid is converted into arsenic acid, under the 
influence of chlorine, which in decomposing water gives 
rise to hydrochloric acid, and the oxygen acting upon the 
arsenious acid transforms it into arsenic. 
The plan pursued consists in dissolving arsenious acid in 
hydrochloric acid, and then diluting this acid with water 
until a given volume of chlorine, say a litre, will transform 
an equal volume of this solution of arsenious acid into 
arsenic. 
Here lies the difficulty which prevents this method from 
being popularised, because is requires much experience in 
manipulation to get exactly a given volume of chlorine in 
an equal bulk of water, none but a Chemist, or those hav- 
ing much practice, could attain this object by Gay Lussac's 
method. One of his plans is to prepare a quart of chlorine? 
