The Pii7'ification of Savage. 201 



cents per head of population per annum, about 45 per cent, 

 of soluble albuminoid ammonia was removed, 90 per cent, 

 of the bacterial germs, and nearly all the suspended matter. 



" With 400 lbs. of ferric oxide in the form of persulphate 

 of iron, costing 44 cents per annum, about 50 per cent, of 

 the soluble albuminoid ammonia was removed, 95 per cent, 

 of the bacteria and all the suspended matter. 



" Taking an equal cost per annum, alum removed 29 per 

 cent, and ferric sulphate 41 per cent, of the total impurity." 



At the time of making these trials, the Massachusetts 

 Board were not acquainted with the most recent develop- 

 ment of the persalts of iron, namely, the basic per- 

 salts of iron ; that is to say, the chloride or sulphate, 

 preferably the latter, charged with an excess of oxide 

 of iron. The oxide of iron is the real agent in the 

 precipitation and oxidation of the organic matter, and 

 hence the advantage of this form of iron compound. 

 The peculiar nature of these compounds causes them to be 

 very readily split up, and their combination with the foul 

 matters of the sewage is therefore very rapid and complete. 

 The precipitate is also rendered heavier, and a clearer effluent 

 without filtration is obtained. The excess of oxide of iron 

 renders the addition of lime almost entirely unnecessary, 

 which, of course, is advantageous, and causes but little solid 

 matter to be added to the weight of the sewage sludge. 

 A striking experiment showing the difference between 

 the basic and neutral persulphate of iron, is the addition of 

 about one cubic centimetre of a solution of these salts to 

 two cylinders of water (about 500 cc. each). The latter 

 simply colours the water, whilst the iron of the former is 

 immediately deposited out of solution principally as ferric 

 hydrate. The sludge from the basic-iron process possesses 

 very considerable immunity from after decomposition, owing 

 to the presence of the hydrated peroxide of iron, which is the 

 only solid matter which finds its way into the sludge. This 



