222 T). Walclie— 0;j the Miuhljj Water of tlie Ilugli. [No. 4<, 



This table shews the very great diilercnce in efficiency between differ- 

 ent substances, common salt having only one-fortieth part of the i)ower of 

 the standard Carbonate of Lime when chemical equivalents are compared, or 

 about one-forty-seventh part when actual weights are compared. On the 

 other hand, Perchloride of Iron is forty times as powerful as Carbonate of 

 Lime, chemical equivalents being compared, or about thirty-six times when 

 actual weights are taken. 



A glance at the table will shew that the precipitating power is just in 

 proportion to the facility with which the acid and basic constituent of the 

 salt can separate. The alkalies and alkaline earths ought to be excluded as ' 

 they exert a chemical change in the soluble constituents of the waters, but it 

 appears to me pretty evident that both the acid and basic constituents of 

 the remainder of these substances take part in the effect produced on the 

 clay. Acids themselves do so, as shewn by the table, even so very weak a one 

 as Carbonic acid gas does so when passed for sometime through the muddy 

 water, as I found from direct experiment. And I also found that when 

 using these small quantities of alumina, the addition of a proportion of 

 potash, more or less, to neutralize the acid constituent of the salt was no im- 

 provement but the reverse. Pieces of sheet iron, immersed in a bottle of 

 muddy water and shaken occasionally, in a few hours caused the mud to pre- 

 cipitate very well ; the iron evidently had been acted on by the Carbonic 

 acid in the water and tiie atmospheric oxygen to form a small quantity of 

 a salt of iron which produced the effect. 



The numbers in the table are by no means to be taken as accurately 

 ascertained. The shortness of the period during which muddy water of 

 nearly similar quality was available rendered this impossible. After the end 

 of August, I employed water from the river mixed in a vessel with the mud 

 deposited from previous water and stirred up, which can scarcely be taken 

 as a very good representative of the water during the worst period of the 

 rains, though probably good enough for the purpose, as the comparisons 

 between different substances were always made with the same water. But 

 as the month of August was chiefly occupied with experiments on the natural 

 constituents of the water, namely alkaline and earthy salts, and those on the 

 effects of the salts of the heavy metals and of the sesquioxides were not made 

 till September when the water had undergone some change, the numbers 

 given for these latter are not quite so certain, possibly ma}- be stated as 

 smaller than they would have been had the August water been used. The 

 decision on this point must be reserved for next rainy season. 



It may be well also to state the absolute quantities of these or at least 

 of some of these substances that would be necessary to clarify a given quan- 

 tity of the muddy water, calculated from the data given. For this purpose 

 the standard will be Carbonate of Lime, dissolved by Carbonic acid, in the 



