1867.] with the supply of water to Calcutta. 7 



Wandle before receiving the Croydon sewage which is 1 .44 grains per 

 gallon, and after receiving it which is 208 grains per gallon. 

 According to this the estimates of organic matter to the extent of 8 or 

 10 grains per gallon in the Hooghly water during May and June shew 

 that it contains fnlly more than the liquid part of the Rugby sewage, 

 and this in a tropical country. 



I do not wish it to be understood that I maintain the perfect accuracy 

 of my own results. The oversight in not examining the samples speedily 

 enough after collection must be admitted, though from all that I have 

 been able to learn from the experiments instituted for the purpose, the 

 error cannot be a great one. There was no great delay in examining 

 the water of the hot season, — about ten or twelve days, and this 

 caused by the time and attention taken up in examining the influence 

 of the tides in numerous samples. There was greater delay with 

 the water of the rainy season, probably about a month with the first 

 samples in July, about a week or 10 days with those of August. 

 This was caused by waiting for the settlement of the very finely 

 divided clay, the presence of which was very unfavourable to the 

 accurate estimation of the organic matter. Recently I have found 

 that the addition of a small quantity of hydrochloric acid causes the 

 mud to settle so rapidly that the water may be filtered clear in course 

 of a few hours : solution of potash or soda and milk of lime do the 

 same, but the water cleared by these reagents seems to contain a 

 different proportion of organic matter than that cleared by simple 

 subsidence. It is of less importance, as the question at present is not 

 respecting the water of the rainy season. The samples of December 

 and February water circumstances prevented me from proceeding with, 

 and they were preserved in stoppered bottles and probably not much 

 done with them till April. The results are consequently more 

 doubtful, though I do not suppose that they are very wide of the 

 truth. As the season advances, should circumstances admit of it, 

 I shall not fail to repeat the analyses, in order to get unobjectionable 

 results. 



The observations made during the last month enable me to add a 

 little to my former statements respecting the effect of the change of 

 seasons on the river water. The increase of organic matter from the 

 rains seems to be chiefly of the more soluble and putrescible kinds ; as 



