1866. | 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 irom 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 Caleutta, 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 spect- 
mens of water when highly concentrated, that of the August water 
being so much deeper in colour than the others. On the contrary, 
a sainple 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 
