Mb. Ceum on Testing SoltUions of Bleaching Powder, 19 



be both tedious and uncertain. The process of M. Marozeau is 

 founded on the property which chlorine possesses of converting 

 calomel, an insoluble substance, into corrosive sublimate, which is 

 abundantly soluble, and which contains twice as much chlorine. 

 Protonitrate of mercury is formed by boiling nitric acid and water 

 with an excess of mercury. It is afterwards diluted and set aside, 

 when subnitrate precipitates. The salt remaining in solution, after 

 being made of a strength to correspond with a volume of dry chlorine 

 gas, is the proof liquor. To ascertain by this means the strength of 

 any solution containing chlorine, we take a measure of nitrate of mer- 

 cury, add muriatic acid to convert it into calomel, and then the 

 chloride slowly. The quantity necessary to make the precipitate 

 entirely re-dissolve is inversely as the chlorine which it contains. 



At last, M. Gay Lussac himself, in the year 1835, announced that, 

 after three years experience of a new process, he had abandoned the 

 method with indigo. His objections to it are partly those already 

 stated, and partly the change which readily takes place on the indigo 

 solution when preserved for any length of time. By the new method, 

 any one of three substances may be employed with the same appara- 

 tus, and with nearly equal advantage — 



1. Arsenious acid. 



2. Ferrocyanide of potassium. 



3. Protonitrate of mercury. 



M. Gay Lussac prefers, however, the arsenious acid, from the preci- 

 sion of its indications. He retains the same basis of measurement as 

 for the test with indigo alone ; that is, he takes for unity the discolour- 

 ing power of one volume of chlorine dissolved in an equal volume of 

 water. That is divided into 100 equal parts. The arsenious solution 

 is prepared of a strength just sufficient to destroy an equal volume of 

 chlorine gas, or of the chlorine solution. If we take a constant 

 quantity of the unknown solution of chlorine, say 10 cubic cen- 

 timeters, and pour into it the arsenious solution till the chlorine is gone, 

 the force of the chlorine solution will be in proportion to the quantity 

 of arsenic employed. If the 100 measures of solution of chloride have 

 taken 100 measures of the arsenious solution, then it has the strength 

 of 100, and it contains its own volume of chlorine gas. If only 80, 

 then it is called of the strength of 80 degrees, and it contains ^^ of 

 its bulk of chlorine gas. But this mode of operating would not give 

 good results, for the muriatic acid which is employed to dissolve the 

 arsenic, and without which the action of the chlorine would be incom- 

 plete, disengages the chlorine from its fixed combination with lime 

 faster than it has arsenic to act upon, and thus a portion escapes into 

 the air. The solution of bleaching powder must therefore be poured 

 by degrees into the arsenious solution, and as the strength of the 

 chlorine solution is then inversely as the quantity employed, a calcula- 

 tion is necessary, or a table has previously to be prepared, by the 



