REPORT ON THE COMPOSITION OF OCEAN-WATER. 
200 
tlieir large excess of surplus base, as carbonate of lime and not as carbonate of magnesia, 
although the latter is the weaker and more abundant base. Reference may here be made to 
certain observations of Sterry Hunt’s, which he made in the course of his experiments on 
the formation of dolomites. He found that a litre of water, containing 3 to 4 grams of 
sulphate of magnesia, can dissolve 1 '2 grams of carbonate of lime, and in addition thereto 
1 gram of carbonate of magnesia, forming a strongly alkaline solution, which, on long 
standing, deposits the whole of its lime as crystals of hydrated carbonate (CaC0 3 5H 2 0). 
Our hypothesis, that a small quantity of free carbonic acid in sea-water enables its 
chloride of sodium to dissolve carbonate of magnesia as a double chloride of magnesium 
and sodium, would explain the local prevalence in the sea of excessive alkalinity, without 
the assumption of the presence of any exceptional proportion of carbonic acid. But the 
presence of excess of carbonic acid, whether the sea be in contact with carbonate of lime 
or with carbonate of magnesia, strongly adds to its tendency towards an increase in the 
alkalinity. The question of alkalinity is, in fact, inseparable from that of the carbonic 
acid in ocean-water, to which we now turn. 
Carbonic Acid in Ocean- Water. 
At the time when the Challenger Expedition set out, it had long been known that 
sea- water does contain carbonic acid (it is obvious that such will be the case, because this 
gas will necessarily be absorbed along with the other gases of the atmosphere), but 
chemists generally were of opinion that this carbonic acid was present for the most part in 
a state of absorption. The presence of carbonates in sea-water would not have been 
denied by any chemist, the less as Bibra, as early as 1851, and others after him, had 
proved that it is alkaline to test paper ; but it had somehow come to be assumed that 
they were present only in minute quantities. This general impression is well illustrated 
by the attitude taken up in regard to this question by Jacobsen, the chemist to the 
German North Sea Expedition * (who has done so much towards rendering our 
knowledge of chemical oceanography more definite), and subsequently by Mr. Buchanan. 
Jacobsen found that while the dissolved nitrogen and oxygen could easily be expelled 
from sea-water by boiling in “vacuo,” the carbonic acid most tenaciously clung to 
the water. On the other hand, he found that the carbonic acid is driven out com- 
pletely when the sea-water (without any addition of reagents) is distilled nearly to dry- 
ness. His explanation of these facts was that the carbonic acid was combined 
chemically, or semi-chemically, with the chloride of magnesium in the sea-water. 
Mere boiling (without sensible evaporation) has no effect upon this quasi- compound ;j 
* Annctlen cler Chemie (for 1873), Bd. clxvii. p. 1 . 
+ According to an experiment of my own, almost all the carbonic acid of a sea-water is expelled without evapora- 
tion, if it is being boiled in a current of ail which takes away the gas as quickly as it is liberated. See page 115, third 
paragraph. 
(PHYS. CHEM. CHALL. EXP. PART I. 1884.) 
A 27 
