976 
THE VOYAGE OF H.M.S. CHALLENGER. 
being a feeble acid, is combined chiefly with the weakest bases, and consequently chiefly 
with the magnesia, and in the second instance with the lime. So we should say, if the 
arrangement of the bases and acids into salts were a mere matter of tendency to form 
simple salts. But magnesium has a characteristic tendency to form double chlorides 
with potassium and sodium, and there is superabundance of chloride of sodium in sea 
water. Hence, probably, most of the magnesium is not there as carbonate but as 
double sodio-cliloride, and the lime takes the greater share of the carbonic acid. The 
alkalinity in any case represents the potential, and may fairly be presumed to measure 
approximately the actual, carbonate of lime. This is the only answer to that often 
raised question about the presence of ready-formed carbonate of lime in sea water, 
which some chemists, who at the time must have deliberately shut their eyes to the 
established propositions of chemistry, have endeavoured to solve by direct experiment. 
Supposing actual carbonate of lime could be extracted from sea water -without the co- 
operation of external matter (I greatly doubt whether this has ever been done), the 
weight of such extracted carbonate of lime could not reasonably be assumed to be equal 
to that which was originally present in the water. Seawater is alkaline, all the alkalinity 
must be owing to carbonates, and of these carbonate of lime must be one. This is, and 
for a time is likely to be, the sum total of our knowledge on this point.” 
At the time when the Challenger Expedition was decided on, and the nature of the 
work to be done in the different departments was being considered, the chemist’s attention 
was principally directed to the gaseous contents of the water, understanding these to mean 
the oxygen, nitrogen, and carbonic acid. 
Dr. Jacobsen, who had been engaged as chemist on board the German ship 
“ Pommerania ” in- her cruise in the North Sea, had found that the method boiling in 
vacuo, which was sufflcient for the extraction of the oxygen and nitrogen, was useless 
for extracting the carbonic acid. He found that, to obtain concordant results, it was 
necessary to distil the water sample almost to dryness and collect the carbonic acid which 
came away with the steam in baryta water or similar absorbent, and determine it thus 
directly. The amount of carbonic acid which Jacobsen thus found (about 88 mgrm. 
per litre) was enormously in excess of what pure water could hold in solution when 
exposed to the atmosphere under similar conditions. At the same time he was unable 
to find in the residue when the water was evaporated to dryness an amount of carbonate 
in any way sufficient to account for the retention of the carbonic acid as bicarbonate. 
Jacobsen was inclined to ascribe to the chloride of magnesium, which is present in 
large quantity in sea water, the property of retaining the carbonic acid. When the 
“Pommerania” touched at Leith on her homeward voyage in August 1872, Jacobsen 
very kindly communicated to Mr. Buchanan all the facts which he had observed, and 
his views as to their possible explanation. 
