•20G 
THE VOYAGE OF H.M.S. CHALLENGER 
which values may be adopted, 'provisionally, for the two kinds of ocean-water. In 
ti ft . 11 rases 1 was in a position to compare with one another the alkalinity of a surface 
water and the bottom water at the same Station. In two cases the balance was in favour 
..f the surface water, the numbers being 0*015 and 0*010 respectively; in one case 
tin- difference was nil; in the remaining twelve cases it was in favour of the bottom 
w it' r, the differences ranging from 0*002 to 0*019. According to the above two averages 
tin alkalinity of bottom water exceeds that of surface water by 0*006, meaning of course 
0*006 grams of carbonic acid per 100 grams of total salts, or 0*014 grams of lime CaO per 
1 00 of chlorine, if we assume the increase in alkalinity to be owing to additional lime. My 
determinations of the lime, as stated, had shown the presence of 0*013 grams of extra 
lime in deep-sea as compared with shallow waters. The closeness of the agreement is 
of course accidental. That the surplus base in a sea- water is not owing entirely to car- 
bonate of lime is too obvious to be specially pointed out. In sea- water (as in any 
mixed salt solution) each base is combined with each acid, and as there are four acids 
and four bases there must be sixteen salts, the individual percentages of which we have 
no means of determining. But there are reasons for assuming that the carbonic acid 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 8odio-chloride, 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- 
■ p. ration of external matter (I greatly doubt whether this has ever been done), the 
weight of -iu Ii extracted carbonate of lime could not reasonably be assumed to be equal 
t<» that which was originally present in the water. Sea-water is alkaline, all the alkalinity 
must be "wing 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. 
h> n I .-aid that the alkalinities in the samples considered ranged on the whole from 
01 40 to 0*164, I meant to hint that these limits do not embrace even all the 130 cases 
admitted for the general discussion. Less values than 0*140, it is true, do not occur; 
but th- r< an -ven cases in which the alkalinity was decidedly greater than 0*164, as 
show'll in tie- following table, in which tie- fir.it column gives the Challenger number of 
