1866. ] with the supply of water to’ Calcutta. 217 
patience, and care ; but it isnot too much to say that the results obtained 
without the above detailed precautions are of no value, or rather worse 
than useless, asthey mislead. The results which I shall give were ob- 
tained by this plan carefully carried out. It will be found that they 
differ materially from those given in Dr. Macnamara’s report; and L 
can only account for the discrepancy, by supposing that some precau- 
tion requisite for ensuring accuracy in the process was omitted, either 
from inadvertance, or because it had not at that time (1862) been 
generally known to or used by chemists. 
In the table in that report the smallest quantity of organic matter 
entered is 0.9 grain in 1 Imperial gallon, the largest 8.3 grains, gene- 
rally however 3 or 4 grains per gallon, which are equal to respectively 
1.23, 11.8 and 4.3 or 5.7 grains per 100,000 grains. My own results 
have yielded me only from 0.6 to 1.9 grains in 100,000 and Dr. Frank- 
land’s in the report already alluded to, vary from .54 to 3.5, or average 
about 1.6 for the Thames, and 1.3 for the other two river waters, The 
table which will be given will exhibit the results I have obtained, 
Remarks will be postponed till the whole subject is considered. 
The time, trouble, and care necessary for estimating the amount of 
organic matter by weight is so great, that chemists have been desirous 
of finding some easier and speedier method of estimating its amount. 
Precipitation of the organic matter by salts of lead or reduction of salts 
of silver and gold have been proposed, but never come into general 
use. But another re-agent has of late been very generally employed, 
the permanganate of potash, which from the facility with which it 
yields its oxygen to organic substances has been made the means of 
estimating the amount of these ; and as it can be very easily employed, 
it has come very much into favour. A good deal of difference of 
opinion prevailed at first as to the proper method of applying it and 
as to the value of its indications, but more agreement is being arrived 
at lately. It is used in the state of weak solution poured from a 
graduated tube, and the permanence of a slight pink tinge in the 
water to which it is added is the sign of the action being complete : 
the quantity by measure of the solution required indicates what is 
wanted. Dr. Letheby continues to add the solution at intervals for 
24 hours: if the action was completed, then this would be very 
well, but it is not, as there are different kinds of organic water, some 
