1806.] with the supply of water to Calcutta. 221 



comparison shews clearly (as might be anticipated) how closely the 

 condition of river waters is connected with the amount of rain-fall ; 

 but, in opposition to the commonly received opinion, it proves that the 

 waters in question are much purer in dry than in wet weather, even 

 if the drought occurs during a very hot summer." He seems, however, 

 to hesitate a little about drawing general conclusions from the observa- 

 tions of one year ; and in the report of the discussion which followed 

 at the Chemical Society's meeting as reported in the Chemical News, 

 some of the speakers seemed inclined to attribute it to special and 

 particular causes. I have no doubt that it is owing to general causes, 

 and that when we consider the circumstances, we cannot expect any 

 other result. 



Unfortunately in the case of the Hooghly at Calcutta, the question 

 is complicated by the admixture of sea water during the hot season. 

 This introduces two sources of error into the process of examination, 

 namely an increased amount of saline matter, and a difference in its 

 nature and properties. These will probably tend to cause indications 

 of an amount of organic matter in excess of the truth. The point is 

 under examination. There is also great difficulty in estimating 

 correctly the amount of organic matter during the rainy season, on 

 account of the impossibility of getting the water clear by filtration, 

 and the very long time it requires to become clear by subsidence. 

 This point is also under investigation.* 



There can be no doubt also that the kind of organic matter in the 

 sea water mixture is different in some respects from that of the river 

 water proper. I was much struck with the observation made many 

 months ago of the difference of colour presented by the different speci- 

 mens of water when highly concentrated, that of the August water 

 being so much deeper in colour than the others. On the contrary, 

 a sample of water from the salt water lake to the east of Calcutta, 

 though indicating both by the weighing and the permanganate 

 processes much more organic matter than the river water, when concen- 

 trated, was almost colourless. 



But to return to the greater proportion of organic matter during the 

 rains, it seems to be nothing but what may be expected. During the 



* For the reasons stated there is considerable uncertainty respecting the cor- 

 rectness of the weight of organic matter in the waters of July and August 



