50 
As a ready method for preparing a dilute solution of 
hypochlorous acid, Gay Lussac recommends to distil a solu- 
tion of bleaching powder with a quantity of dilute nitric 
acid which is just sufficient to liberate the hypochlorous 
acid. 
According to Gay Lussac’s view, bleaching powder is a 
mixture of calcium chloride and calcium hypochlorite, and 
the same view is held by most chemists. Professor Odling 
has however pointed out that, calcium being a dyad metal, 
r Cl 
the constitution of bleaching powder was probably Ca | 
or it was at the same time a hypochlorite and a chloride. Of 
course both views explain equally well the formation of 
hypochlorous acid by Gay Lussac’s method. I read there- 
fore with great surprise a paper by Goepner (Dingler’s 
Polytechn. Journ, 209, 204), in which he states that bleach- 
ing powder is nothing but a simple combination of lime and 
chlorine, which by acids is again resolved into its consti- 
tuents without the least trace of hypochlorous acid being 
formed. He says that, although the preparation of hypo- 
chlorous acid by this method is described in all hand books 
as if this experiment had been made hundreds of times, this 
is a mistake, and the reason why this error has maintained 
itself so long in chemical literature is that hitherto no 
reaction was known by which free chlorine and hypochlo- 
rous acid could be readily distinguished. But such a re- 
action has now been found by Wolters, who has shown that 
when chlorine-water is shaken with an excess of mercury 
only mercurous chloride is formed, while with aqueous 
hvpochlorous acid it yields a brown crystalline oxychloride 
of mercury, which is readily soluble in hydrochloric acid, 
and thus offers a ready means of the qualitative as well as 
quantitative determination of hypochlorous acid in the pre- 
sence of free chlorine. 
