2 9 2 



NA 7 (J RE 



[July 24, 1884 



it will be seen that Moigno's mind was of a very 

 practical cast, and that he was not immersed in the 

 consideration of theories to the neglect of what is more 

 useful. 



THE COMPOSITION OF OCEAN WATER 1 

 I. 



ALTHOUGH ostensibly a report on the composition 

 of ocean water, this memoir includes in its 250 large 

 quarto pages the record of a far more extensive research 

 than the title implies. It contains a detailed account of 

 seventy-seven complete analyses of sea water, largely 

 accomplished by the use of new and specially invented 

 methods, the record of several independent researches 

 into purely theoretical matters, and a number of exhaus- 

 tive experimental criticisms of methods employed in 

 similar work by other chemists. Taken altogether, the 

 Report reads like the account of a life-work, and it is 

 wonderful how the immense amount of work described 

 in it could possibly be accomplished in the six years 

 which have elapsed since the return of the Expedition. 

 The rapid completion of the work is in great measure 

 due to Prof. Dittmar's custom of having all the routine 

 determinations made by assistants under his immediate 

 supervision, while he devoted himself specially to the 

 invention and trial of new methods and the repetition of 

 doubtful experiments. The gentlemen who assisted in 

 the research, and whose services Prof. Dittmar is scrupu- 

 lously careful in acknowledging, are Messrs. John 

 M'Arthur, Robert Lennox, Thomas Barbour, W. G. 

 Johnston, James M. Bowie, James B. M'Arthur, G. A. 

 Darling, and Moses T. Buchanan. 



What first strikes a chemical reader on looking through 

 the volume is the essentially mathematical treatment of 

 the whole subject. The value of the statistical method 

 in discussing experimental results has been gradually- 

 realised by chemists, but it is questionable if it has ever 

 been applied more fully or with more satisfactory effect 

 than here. The first care in every case, after taking all 

 possible precautions to insure the utmost attainable accu- 

 racy, was to ascertain the limiting values of the probable 

 error of the analytical method, and for this purpose there 

 were never less than two and frequently more than four 

 determinations made of each constituent. The utmost 

 pains has been taken to represent the numerical results in 

 as many aspects as possible, in tables, in mathematical 

 formulae, and by means of curves. 



It is only possible here to indicate the principal con- 

 tents of the six chapters into which the memoir is divided. 

 The consideration of Chapter II. "On the Salinity of 

 Ocean Water," may be conveniently reserved for a sub- 

 sequent article, where it will be taken up along with Mr. 

 Buchanan's report on the specific gravity of ocean water, 

 which forms Part II. of the volume. 



Although sea water had been subjected to many analy- 

 ses in the earlier part of the present century, the only 

 research of permanent importance until very recently was 

 that of Forchhammer, who analysed a great number of 

 surface waters from all parts of the ocean in 1864. Prof. 

 Di ttmar avowedly took this research as aguide, and intended 

 his work to be merely supplementary to it ; but from the 

 circumstances of the two chemists the later work tends 

 rather to supersede than to supplement the earlier. 

 Forchhammer dealt with surface water only, collected 

 and brought home in corked bottles by seafaring men 

 who, however willing to do their best, could not be alto- 

 gether trusted to observe requisite precautions, while 

 Dittmar was supplied with water from all depths of the 

 ocean collected at exactly known positions under the 



1 "The Physics and Chemistry of the Voyage of H.M S. Challenger. 

 Vol. i. Part i. Report on the Composition of Ocean Water" By Prof. W. 

 Dittmar, F.R.SS.L. and E. 



constant supervision of Mr. Buchanan, who secured each 

 sample as it was drawn in a carefully stoppered bottle. 

 We must take into account also the greater delicacy of 

 the balances, and the more perfect analytical methods 

 which are now available. The following table quoted 

 from p. 203 of the Report, shows the most recent numbers 

 assigned to the components of ocean-water salts compared 

 with those given by Forchhammer : — 



Dittmar 



Chlorine 55 '292 



Bromine 0-1884 



Sulphuric acid (SO.,). 6-410 



Carbonic acid (C0 2 ).. 0-152 



Lime (CaO) 1-676 



Magnesia (MgO) ... 6-209 



Potash (K,0) 1-332 



Soda (Na 2 b) 41 -234 



(Basic oxygen equiva- , _ 



lent to the halogens) 1 '"•' 



Per 100 parts of halogen 



calculated as chlorine. 



Dittmar Forchhammer 



99-848 ... not determined 



0-34.02 ... ,, 



11-576 ... u-88 



0-2742 ... not determined 



3-026 ... 2-93 



11-212 ... 11-03 



2-405 ... 1-93 



74-462 ... not determined 



Total salts 



ioo-ooo 



180-584 



More than thirty elements are known to exist in solution 

 in the ocean, but most of these are present in such minute 

 quantity that it was hopeless to attempt to determine them 

 in a number of small samples. Attention was accordingly 

 confined to the chlorine, sulphuric acid, soda, potash, 

 lime, and magnesia, which were estimated with very great 

 accuracy and always by the same method, so that if more 

 exact processes should be discovered at any future time 

 the error of the method used may be calculated once for 

 all, and applied as a correction to each analysis. 



This rule of rigid adherence to one system was broken 

 through in one case, that of the potash, where the ordinary 

 process, which was first adopted, proved so unsatisfactory 

 that it was worse than useless to continue to employ it, 

 and the later analyses were conducted by a modification 

 of Finkener's method that gave better results, through a 

 curious balancing of the errors. 



For the particular methods employed in each case it 

 is necessary to refer to the memoir itself, where they are 

 described with the utmost detail ; but reference must be 

 made to the great improvement which Prof. Dittmar has 

 introduced in what was formerly called volumetric ana- 

 lysis, but which he now prefers to name tiirimetric. It 

 may be defined, somewhat paradoxically perhaps, as 

 volumetric analysis by weight. The standard solutions 

 are made up as usual by weighing the salt and measuring 

 the water, but the whole solution is afterwards weighed, 

 and its strength is thus determined with great accuracy. 

 A balance combining strength and delicacy to an unusual 

 degree is of course necessary for this purpose. By per- 

 forming the titration in a weighed phial containing a 

 weighed amount of liquid, and weighing it again after the 

 reagent has been added to the proper amount, the burette 

 error is obviated, except in those cases where the method 

 of zig-zag titration is adopted, and then it only affects the 

 measurements of the few drops of each reagent that are 

 added in turn to produce and destroy the coloration which 

 marks the end-point. All the chlorine determinations 

 were made in this way by Volhard's method of precipitat- 

 ing the halogen by excess of silver nitrate, and estimating 

 this excess by means of a standard solution of ammonium 

 sulphocyanate in presence of iron alum. 



The result of the seventy-seven complete analyses of 

 ocean water, the description and discussion of which 

 forms Chapter I., confirms Forchhammer's discovery that 

 the percentage composition of the salts of sea water is 

 the same in all parts of the ocean, and extends it to water 

 from all depths. The application of the principle of con- 

 stant composition to depth is subject to a slight but very 

 important exception. The proportion of lime was found 

 by Dittmar to be greater in very deep water than in that 

 near the surface. Although the difference found exceeded 



