124 Mr. Waldies Investigations connected [No. 2, 



which receives the greater part of the sewerage of Calcutta requires 

 less oxygen to destroy the products of putrefaction than the best tank 

 waters of Calcutta ; and the water of the salt marsh to the east of the 

 town, called the Salt Water Lake, requires only about as much as that 

 of General's Tank of the same age ; for taking the rate of improvement 

 between 3 hours and 26 hours, General's Tank water at 19 hours old 

 would require .1626 grain oxygen. Results like these have led me 

 to set but a small value on this favourite process, and induced me to 

 turn to others promising more trustworthy indications. 



The fundamental point with respect to the organic matter is the 

 same as that connected with any other constituent, namely, its propor- 

 tion by weight, ascertained as accurately as practicable. The method 

 of doing this has already been detailed in the first paper, and I have 

 only to repeat that, with a fine balance, patience, and care, it gives fairly 

 satisfactory results. Attention to details is advisable to procure 

 uniform results. Of the ordinary river or tank waters, I usually 

 evaporate from 10,000 to 40,000 grains, according to the kind of water, 

 contriving so as to have 4 to 6 grains of dry residue, beside 3 grains 

 of dry Carbonate of Soda* added to the water, when put to evaporate. 

 At one time I did not use the soda for some kinds of water, as 

 unnecessary, but now I use it always. It makes the results more 

 accurately comparable. These quantities are sufficiently large for the 

 crucible, which holds conveniently about 200 grains of carbonic acid 

 water, but requires to be twice or thrice filled up. A larger crucible 

 would be more convenient, in which case once might do. 



The river water of the cold season of 1865-66 had been kept over 

 from two to four months, and the results as to organic matter therefore 

 were doubtful. These will now be replaced by new determinations, all 

 made without delay. There has been no opportunity yet for making 

 new determinations of the hot season and rainy season waters ; but I 

 have already, in the u Supplementary Observations," given reason for 

 believing that the delay of from 9 to 16 days in making these 

 determinations, in the case of the hot season waters, cannot have been 

 productive af any serious error. Additional reasons will be given for 

 this opinion presently. There is greater doubt respecting the July and 



* Dr. Parkes in his " Practical Hygiene" recommends 30 grains Carb. of 

 Soda ! This is surely a misprint. He also recommends to restore the carbonic 

 acid lost by ignition by adding solution of carbonic acid or carbonate of ammenia, 

 This is a mistake : the results by carbonate of ammonia are totally wrong. 



