1867.J with the supply of water to Calcutta. 121 



those that are most easily oxidized. These, it appears to me, are rather 

 weak grounds on which to found the preference which is at present given 

 to this mode of estimating the" degree of organic impurity in water. 

 Others speak of it as indicating the amount of putridity in the water, 

 and this, in my opinion, comes nearer the truth. By this I understand 

 that the amount of oxygen required is in proportion to the amount of 

 certain products of the putrefactive fermentation of the organic matter 

 in the water. This, however, as Dr. Frankland has stated,* furnishes 

 no indication of the amount of organic matter actually present in the 

 water. The offensive smell and other properties of these products 

 make it more than probable that they ■are injurious to health ; but 

 even then it is not certain that there may not be other constituents, 

 equally or even more injurious, but more difficult of oxidation. 

 Nor is it even certain that these products of putrefaction are the only 

 substances which are readily oxidized by the permanganate. 



Moreover, a portion of these products are evidently of a very 

 unstable character and quickly disappear, or at least lose their power of 

 deoxidizing the permanganate. This was first brought particularly 

 to my attention by the objections raised to my determinations of 

 organic matter in the original paper, and has been noticed in the 

 supplementary observations. Since then, I have made numerous 

 observations on this point, and give a few selected ones by way of 

 illustration. The details of the mode of observation are given in the 

 original paper. 



* Chemical News, March 23, 1866. 



16 



