''CHALLENGER" CHEMISTRY AND PHYSICS. 109 



The last column gives the average composition of a sample of ocean-water of 

 specific gravity 1027. 



Professor Dittmar's work fully confirmed Forchhammer's con- 

 clusion as to the constancy in composition of the sea-water in various 

 parts of the ocean, and further showed that the depth is without 

 influence on the composition of the salt, if we limit ourselves to the 

 ratios between chlorine, sulphuric acid, magnesia, potash and 

 bromine. This last constituent, occurring as it does in very small 

 quantities, and being considerably utilised by sea-plants, was con- 

 sidered by Dittmar to be more liable to local variation than the 

 others. From the determinations of the lime, however, there would 

 seem to be no doubt that its proportion increases with the depth. 

 This result of the separate analyses was confirmed by the analysis 

 of mixtures of superficial and deep waters, and also by the alkalinity 

 determinations, deep waters almost always showing a higher alkalinity 

 than superficial ones. Let us explain what is meant by alkalinity. 

 All sea-water is alkaline to test paper^ and reagents, and as Dittmar's 

 complete analyses always showed an excess of equivalents of base 

 over those of sulphuric and hydrochloric acids, all this alkalinity must 

 be due to the base united with carbonic acid. Dittmar (after Tornoe) 

 measured this alkalinity by determining how much carbonic acid was 

 required to form normal carbonates {i.e., similar to carbonate of lime) 

 with the base of the sea-water not united with sulphuric and hydro- 

 chloric acids. Carbonic acid being a weak acid is probably combined 

 chiefly with the weakest bases — that is, with the magnesia and lime ; 

 and the magnesium having a tendency to form double salts, probably 

 forms a double chloride with the abundantly present sodium chloride, 

 leaving the greater share of the carbonic acid to the lime. At any 

 rate, the " alkalinity " measures the potential carbonate of lime, and 

 may fairly be presumed to measure approximately the actual carbonate 

 of lime present. Thus the higher alkalinity of the deeper waters 

 argued a higher percentage of carbonate of lime. 



The gaseous contents of sea-water — oxygen, nitrogen, and 

 carbonic acid — are of great importance to the biologist, for, in view of 



1 This was observed as early as 1777 by Bergmann ; Von Bibra (1S51), although 

 an independent discoverer of the fact, was certainly not the first, as supposed by 

 Tornoe and Dittmar. 



