OF WATER FOR ORGANIC MATTER. 47 



lime and some magnesia witli them ; and on the whole 

 the great change to be observed in the water is connected 

 chiefly with the alkaline salts. The most stable and most 

 constant of these, without loss, is common salt. 



On this subject I may quote what has been said elsewhere 

 regarding the chlorides as an indication of the presence of 

 animal matter, proper regard being had to the absence of 

 other sources, which in this country are chiefly connected 

 with artificial circumstances. " The absence of the chlo- 

 rides, especially, may be considered as the certain absence 

 of sewage matter, and their amount to be estimated as an 

 indication of the comparative amount of sewage matter 

 from, day to day in cases where sewage only pollutes the 

 river/'' I conclude from these data that the organic matter 

 and the nitrogen in it are equally incapable of giving us a 

 sufficient history of the water. We may draw another 

 conclusion, that nature has provided a mode in certain 

 conditions for its perfect removal. 



It is worth while to ask whether this purification by 

 putrefaction is not the most complete method ; it seems to 

 destroy that which we consider to be germs, which germs 

 may be a beginning of any disease of the most terrible form. 

 If the water were allowed to purify itself by standing, and 

 producing whatever amount of organic life could be deve- 

 loped from it, the germs would be increased in number, in- 

 stead of being diminished. The water above might remain 

 very clear, but might be nevertheless very dangerous. In 

 such a condition water seems to be best purified by filtra- 

 tion through the soil, which simply means extremely slow 

 and careful filtration accompanied with partial oxidation ; 

 in that case the water coming from under the soil would 

 be fit again for use, although it would contain, and does 

 contain, the alkaline salts alluded to. 



Waters from rich lands, unless from very deep drains, 

 always contain more organic matter than water from hills. 



