504 THE DEPTHS OF THE SEA. [CHAP. x. 
carbonic acid was first absorbed by a strong solution of caustic 
potash; and subsequently the oxygen was absorbed by the 
addition of pyrogallic acid, the remaining gas being assumed to 
be nitrogen. 
The results of the analyses were always corrected to the 
standard temperature of 0° Cent., and to 760 millimetres’ baro- 
metric pressure, for comparison among themselves and with 
others. In nearly every case the duplicate analyses from the 
same gaseous mixture agreed closely, if they were not identical. 
(3) The examination of the sea-water for organic matter was 
made according to the method detailed by Prof. Miller in the 
Journal of the Chemical Society for May 1865, with an addition 
suggested by Dr. Angus Smith. Each sample of water was 
divided into two; to one of these a little free acid was added, 
and to both an excess of a standard solution of permanganate 
of potash. At the end of three hours the reaction was stopped 
by the addition of iodide of potassinm and starch, and the 
excess of permanganate estimated by a standard solution of 
hyposulphide of soda. The portion to which free acid was added 
gave the oxygen required to oxidize the decomposed and easily 
decomposable organic matter; the second portion gave the 
oxygen required by the decomposed organic matter alone, which 
was usually from about one-half to one-third of the whole. 
The following is a summary of the total number of obser- 
vations, analyses, &c., made during the three cruises respec- 
tively :— 
| First | Second} Third Total. | 
ernise. | cruise. | cruise. 
Specific-gravity determinations .| 72 27 26 | 125 
| 
Duplicate gas-analyses . . . .| 45 | 28 21 89 
Organic-matter tests. . . . .| 137 96 | 32 | 195 
Specific Gravity—The specific gravity of surface-water was 
found to diminish slightly as land was approached; but the 
« 
