982 
THE VOYAGE OF H.M.S. CHALLENGER, 
Temperature, 
°Cl 
C0 2 Milli- 
grammes per 
Litre. 
Remarks. 
54-0 
63-9 
54-0 
80-0 
Freshly drawn. 
26-0 
14-5 
Allowed to stand till the temperature fell from 54° to 26° C. 
22-0 
o-o 
Allowed to stand two hours outside of port, while temperature fell to 22° C. 
31-5 
48-5 
From hold tank. 
22-0 (?) 
o-o 
The same allowed to stand some time outside of port. 
53-0 
65-7 
Fresh from condenser. 
29-2 
80-0 
Fresh from hold tank. 
18-3 
o-o 
The same allowed to stand outside port. 
The amount of carbonic acid in the freshly collected distilled water is thus very large, 
notwithstanding the high temperature of the water. The unstable condition of the 
mixture is shown by the rapidity and completeness with which the carbonic acid disappears 
when exposed to the air. The results show also in a very evident way that in sea water 
there can be no appreciable amount of really free carbonic acid. If much more be found 
than is required for the formation of bicarbonate, then it is retained by some other agency 
than the absorptive power of the water itself. 
With regard to the dissociation tension of carbonic acid in sea water, Professor 
Dittmar writes : 1 — 
“ Considering that at a temperature of 18° to 21° C. the dissociation tension of the 
bicarbonates in sea water is 5 ten-thousandths of an atmosphere, at temperatures not 
differing by more than one or two degrees from 0° C., such as prevail in the Arctic and 
Antarctic Regions, it is far more likely than not to fall below 3 ten-thousandths, which 
is about the partial tension of the carbonic acid in the atmosphere. Admitting this, 
and assuming that at a given time the ocean everywhere contained its surplus base as 
sesqui-carbonate, then the water of the tropics would constantly give out carbonic 
acid to the atmosphere, and tend to raise its 0'0003 atmosphere .of carbonic acid 
pressure to the dissociation tension corresponding to the temperature. Passing now 
from the Equator, either way, to colder and colder latitudes, this carbonic acid emis- 
sion becomes less and less intense, until, in a certain belt of temperature which 
prescribes to the dissociation tension the value 0'0003, this emission becomes nil, 
1 Phys. Ghem. Chall. Exp., part i. p. 212, 1884, 
