476 On Filtering the Waters of Tanks. [No. 6. 



in the country in England a hard water, i. e. it holds a considerable 

 proportion of sulphate of lime in solution ; curdling soap even after it 

 has been boiled, and is even said to affect the bowels of persons un- 

 accustomed to use it. To correct this defect, the French use in their 

 cisterns and filters what are called Boules de Mars which are merely 

 an earthy red oxide of iron, and these decompose the sulphate of lime 

 and render the water much better for domestic use. The white clays 

 to be found in many places in India, and which all contain alumina, 

 and some of them sulphate of lime (gypsum) will also frequently be 

 found useful as chemical filters. 



It is evident from what has been said above, that the mere clearing 

 materials which serve to render the water limpid, and those which act 

 chemically may be combined either in mixtures or in layers as thus. 

 In ten feet of filter there might be five beds of coarse sand, and five 

 of iron kunkur or chalk, or those materials may be kept separate in 

 the two divisions of the filter, and thus that, by varying the filtering 

 media, we may with great probability assert that we might obtain 

 nearly pure water at a very small expense, whenever the Government 

 or the Municipality, or any individual who can afford it, will undertake 

 the cost of the experiments on a proper scale. If a long narrow canal 

 was led from a tank to any convenient situation for a reservoir and 

 different divisions, say at every ten or fifteen feet, were filled with 

 different kinds of filtering materials, this would be the same process 

 in a horizontal direction ; but not, I think, so efficacious as that 

 which I have proposed, upwards and downwards. 



H. PlDDINGTON. 



May \sl, 1852. 



