212 D. Waldie— O^ the Muddy Water of the Rugli. [No. 4, 



sition by gravity, on the upper surface of these granules, of still finer parti- 

 cles ;* and last, and least important of all, is the mutual attraction of particles 

 of mud and sand independent of gravity. The influence of all of these pro- 

 cesses will be affected by the greater or smaller size of the particles of mud 

 in suspension, and the difficulty in getting the water to pass clear will be 

 the greater the smaller the particles are. Hence the great difficulty with 

 the water in question, the particles being so very fine. 



II. — The peculiarity of the Hugli water from June to October I 

 had always connected with the tropical rains and melting of the snows at the 

 sources of the Ganges, occurring together so as to form one great flood of 

 four or five months duration, instead of occasional floods to which rivers of 

 European countries are subject, exceeding these greatly both in amount 

 and in duration. I had not, however, been able to offer any other than 

 conjectural explanations of its slowness in clearing by settling and of the 

 difficulty in filtering it, of which I had found abundant evidence as a matter 

 of fact. Attention having been again directed toward the subject by the 

 circumstances previously referred to, I was led again to think of the advan- 

 tage it would give me to be able to state some reason for my belief that the 

 cause of difficulty lay in the nature of the water ; some explanation of this 

 peculiarity, — a reason why it should be so, — some generalisation shewing 

 that it was not an isolated fact, but one of other similar facts admitted and 

 acknowledged. Indeed, ever since it had been so forcibly brought under my 

 own notice by my filtration experiments, and fixed in my own mind at least 

 as a certainty, I had been alive to everything, old and new, that appeared to 

 have a bearing on the subject, whether it were practicable means of purifying 

 the water or a way of explaining the difficulty. 



With respect to purifying muddy water generally there were certain 

 methods which were well known and others less generally. The use of alum 

 and other salts of alumina for such a purpose had long been known : salts of 

 peroxide of iron, a substance chemically having much analogy with alumina, 

 had more recently been introduced, and in my opinion they were even superior 

 to salts of alumina. These substances act by the alumina or peroxide of iron 

 being separated from its combination with the acid by alkaline matter which 

 might be added along with them, or by the action of the carbonate of lime 

 present in many waters, or even simply by large dilution, as in this case the 

 base tends to separate from the acid. The alumina or oxide of iron separates 

 in loose soft flakes which envelop or attract the fine particles of the mud, 

 and carry them down with them, leaving the liquid quite clear. Indeed many 

 other substances that produce flocculent precipitates by the addition of 

 another substance have the same effect ; thus by adding a solution of sulphate 



* Well illustrated by Wauklyu and Chapman in the 2nd edition of their treatise 

 on Water Analysis. 



