CONSTITUTION OF BLEACHING-POWDER. 267 



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 hypochlorous acid could be readily dis- 

 tinguished. But such a reaction 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 hypochlorous acid it yields a 

 brown crystalline oxy chloride 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 presence of free chlorine. 



In employing this reaction for detecting hypochlorous 

 acid in the liquid which was obtained by distilling bleach- 

 ing-powder with a small quantity of hydrochloric or sul- 

 phuric acid, Goepner could not find a trace of hypochtorous 

 acid, but only free chlorine. 



I have already mentioned that he says the preparation 

 of hypochlorous acid by this method is described in the 

 books as if this experiment had been repeated hundreds of 

 times. Now this experiment has been repeated many 

 hundred times in our laboratory only. Professor Roscoe 

 shows it every year in his lectures ; and all our students, 

 in the course of their practical work, perform it, and find 

 that the perfectly colourless distillate is a much more 

 powerful bleaching -agent than freshly prepared chlorine- 

 water. This is quite sufficient to show that the liquid 

 contains hypochlorous acid. But why did Goepner fail in 

 detecting it? Perhaps it was the fault of the analytical 

 method. To decide these questions I prepared hypochlo- 

 rous acid by distilling solutions of bleaching-powder with 

 dilute nitric and sulphuric acid, and shook the colourless 

 distillates with mercury. In every case the brown oxy- 

 chloride was formed in quantity, and possessed all the 

 properties which Wolters has assigned to it ; while hy 



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