1873.] D. Waldie— O^i tie Muddy Water of the Hugli. 213 



of copper, and then a little soda, oxide of copper is thrown down carrying 

 the mud with it, I have, indeed, made use of this method for precipitating 

 the fine, suspended mud for chemical examination ; the oxide of copper 

 being removed from the precipitate, after collection, by ammonia and acetic 

 acid, and the mud washed. This process, when the precipitants are employ- 

 ed in proper quantity, is speedy and convenient. 



There is another class of substances which operate in a similar way, 

 namely, alkalies and alkaline earths, such as Soda and Lime. These com- 

 bine with the carbonic acid that keeps carbonate of lime in solution, which 

 becoming insoluble is consequently precipitated. When Lime is used an 

 additional quantity of carbonate of lime is produced. This, in fact, is Dr. 

 Clark's well-known process for softening such waters as owe their hardness 

 to carbonate of lime in solution. The precipitate formed carries down other 

 matters with it leaving the water clear. The objection to the use of this 

 process is the large quantity of additional sediment produced, and the risk 

 of some prejudicial effect on the quality of the water, at least if not carefully 

 managed. 



Another class of substances the mode of action of which is not so evident, 

 is acids. I do not know when this was first noticed. G-raham, Miller, and 

 Hofmann in their Report on the London waters, June 1851, speaking of the 

 impurities, refer to " this clay tinge which resists the action of acids." 

 Whether from this hint or not, I do not recollect, but I myself employed 

 acids in 18G6 for the purpose of clarifying the muddy waters of the Hugli 

 during the rains.* A small quantity of Nitric or Hydrochloric acid added to 

 a large bottle of muddy water so altered and precipitated the mud that next 

 day, or even in a few hours, it could be filtered clear with ease. I used alka- 

 lies, also, and perchloride of iron ; but did not prosecute the subject further, 

 my object having been simply to get the water clear with as little addition 

 of foreign matter as possible ; and nothing was better than a little of these 

 acids, — even of acetic acid. 



There is yet another class of substances the action of which is equally 

 if not more difficult to explain, namely, those substances usually called neu- 

 tral salts, both alkaline and earthy. The first direct notice I found of this 

 was in some remarks in the ' Chemical News' of 3rd April, 1868, by Mr. W. 

 Skey, Chemist to the Geological Survey of New Zealand, on the property 

 of this class of substances to clarify muddy water. In this he specifies that 

 1 grain of common salt clarifies 5 ounces of muddy water and 1 grain of 

 chloride of calcium or barium 10 ounces, 1 grain of lime 15 ounces and 1 

 grain of sulphuric acid 50 ounces. He thinks that these substances must 

 act solely from their afiinities for water, as it is not at all likely that they 

 undergo any decomposition themselves. In the ' Chemical News' of 8tli 

 * Journ. As. Soc. Bcng., 1867, Vol. XXXVI, Pt. II, p. 7. 



