220 M. Gay-Lussac on the Chloride of Lime. [Sept. 



chloride, the advantage of manufacturing the sub-chloride only 

 is obvious ; its preservation and transport are much more easily 

 effected. 



The quantity of chlorine in combination with water, or a base, 

 may be estimated by several processes ; but in the arts, in 

 which dispatch is important, the preference has been given to 

 M. Descroizilles' process, founded on the property of chlorine 

 to discolour indigo. One part of indigo dissolved in 9 parts of 

 concentrated sulphuric acid, and then diluted with 990 parts of 

 water, forms the coloured liquid usually employed to ascertain 

 the quality of the chlorine. 



Under the same circumstances, chloride of lime discolours a 

 quantity of this solution proportionate to its own ; but if they 

 vary, the results also are very variable. Thus, if we pour the 

 chloride slowly into the indigo, a much smaller quantity of it 

 is necessary to effect the discoloration than if we proceed dif- 

 ferently. The minimum of discolouring effect, is obtained by 

 pouring the indigo very slowly into the chloride, and the max- 

 imum by pouring the chloride very slowly into the indigo. Re- 

 peated trials have proved that the best process for obtaining 

 constant and comparable effects, is to pour the solution of in- 

 digo rapidly into the solution of chloride, or the latter into the 

 former. I shall explain the mode of operating by and bye. 



If the indigo of commerce were pure, or always of the same 

 quality, the quantity of its solution employed in each assay 

 would give the relative quality of the chloride ; but since its 

 quality is very variable, the results of trials made with different 

 indigos cannot be compared together. To avoid these incon- 

 veniences, I have followed the example of M. Welter, and taken 

 as unity of discolouring power that of pure, dry, chlorine, at the 

 barometrical pressure of 0'76m. (29*92 inches,) and temperature 

 of 0°. (32 Faht.) I prepare a solution of any of the best indigos 

 of commerce of such a strength that the chlorine discolours 

 exactly ten times its volume of it, and I call this solution the 

 proof tincture ; and each volume of proof tincture that is dis- 

 coloured I call a degree, and I divide the degree into ten parts. 



Thus, if we take 10 grammes* of chloride of lime and dis- 

 solve it in such a quantity of water as to form 1 litre of solution, 

 the number of degrees, or volumes of indigo discoloured by 

 one volume of the solution of chloride, will indicate the number 

 of tenths of a litre of chlorine that the solution contains. Con- 

 sequently, 1 kilogramme f of chloride of lime, whose quality 

 had been determined by this method, and found to be of 7*6° 

 or y^jj-ths, would contain 76 litres of chlorine. Each degree 

 therefore is equal to 10 litres, per kilogramme of chloride, and 

 each tenth of a degree to 1 litre. Supposing the sub-chloride 



* Or 1 decagramme, Tr. + Or 100 decagrammes, Tr. 



