July 24, 1884] 



NA TURE 



= 93 



the sum of all the probable errors of analysis, it was con- 

 sidered necessary to apply a more stringent test in order 

 to make sure that the increase in lime was really in rela- 

 tion to the depth. For this purpose three mixtures were 

 made, each of about seventy samples of water from all 

 parts of the ocean, but the first consisting entirely of 

 surface waters, the second of samples from between 300 

 and 1000 fathoms, and the third of waters from a greater 

 depth. The exact analysis of these proved beyond ques- 

 tion that the proportion of lime increases with the depth. 

 The same bathymetrical mixtures were used for deter- 

 mining the bromine, as it, of all the minor components, 

 is the one which might be supposed to vary most with the 

 depth, owing to the fact of its being so largely absorbed 

 by marine vegetation ; but the proportion was found to 

 be invariable within the limits of error. The details of 

 this very difficult and interesting series of experiments 

 occupy Chapter III. 



The question of the amount of carbonic acid in sea water 

 is one which cannot even yet be said to be definitely settled. 

 The simple and elegant adaptation of Tornoe'smodification 

 of Berchert's and Classen's apparatus, which Prof. Dittmar 

 made, and which is figured in the memoir, gets over the 

 difficulty of determining the total carbonic acid in sea 

 water ; but unfortunately it had not been invented when 

 the Challenger sailed. The daily determinations of car- 

 bonic acid in sea water which Mr. Buchanan made were 

 performed by a method that only took account of what 

 must be called, for lack of a better expression, the loosely- 

 combined carbonic acid ; that is, the portion of the gas 

 existing in a state of absorption in the water, and the part 

 combined with the normal carbonates to form bicar- 

 bonates. The immense number of determinations, made 

 in the same way under exceptionally favourable condi- 

 tions, form a valuable series for purposes of comparison, 

 and Prof. Dittmar has not failed to utilise it. His critical 

 experiments on Mr. Buchanan's method made with the 

 view of ascertaining its limits of uncertainty were, he 

 acknowledges, insufficient for the purpose. This is to be 

 regretted, for an exhaustive series of carbonic acid deter- 

 minations performed on the same water by the two forms 

 of apparatus under favourable conditions might be ex- 

 pected to produce valuable results. Prof. Dittmar saw 

 that it was useless to employ samples of Challenger water 

 which had been kept for several years in order to esti- 

 mate the total carbonic acid. But instead of giving up 

 the research on this account he proceeded by an ingenious 

 use of synthetic sea waters to study the behaviour of 

 bicarbonates in solution. He says (p. 212) : — 



" I am aware that this part of my work lacks the degree 

 of precision which would be desirable for my present train 

 of reasoning. But I had not the time to embark in the 

 far more elaborate investigation which would have been 

 desirable. I have, however, quite lately resumed the 

 matter on a new basis, and hope before long to be able to 

 formulate the exact conditions of stability in sea-water 

 bicarbonates as they exist when dissolved in real sea 

 water, and amongst others to decide the question whether 

 in this process they quite directly tend to become normal, 

 and do not perhaps more directly gravitate towards the 

 state of sesquicarbonate. In the meantime we must 

 reason on what data we have." 



And reasoning on these data he produces a most inter- 

 esting theoretical disquisition on the dissociation-tension 

 of dissolved bicarbonates at various temperatures, and 

 shows how the ocean acts as the great regulator of atmo- 

 spheric carbonic acid. The three main results of the 

 Challenger observations on this subject are given (p. 

 220) as : — 



" 1. Free carbonic acid in sea waters is the exception. 

 As a rule the carbonic acid is less than the proportion 

 corresponding to bicarbonate. 



"2. In surface waters the proportion of carbonic acid 

 increases when the temperature falls, and vice versa. 



" 3. Within equal ranges of temperature it seems to be 

 lower in the surface water of the Pacific than it is in the 

 surface water of the Atlantic Ocean." 



In relation to Mr. Murray's theory of coral-reef forma- 

 tion and of oceanic deposits generally, it would appear 

 probable that bottom waters contained more carbonic 

 acid than those near the surface, and that this carbonic 

 acid was the agent which dissolved the calcium carbonate 

 of shells at great depths. Prof. Dittmar thinks otherwise. 

 In his opinion Mr. Buchanan's numbers prove bottom 

 water to contain no more carbonic acid than surface 

 waters, and he supposes that the solution is effected by 

 prolonged contact with the sea water itself, for by experi- 

 ment he found that it was capable of dissolving calcium 

 carbonate, though very slowly. It is not quite plain that 

 Mr. Buchanan's numbers do warrant this conclusion, and 

 there seems to be room for further research in this 

 direction. 



Chapter V. treats of the alkalinity of ocean water. The 

 seventy-seven complete analyses showed that in sea-water 

 salts there is a distinct preponderance of base over fixed 

 acid, the difference being presumably due to carbonates ; 

 and the direct determination of the extra base by standard 

 hydrochloric acid at the boiling point, after the method 

 devised by the chemists of the Norwegian North Atlantic 

 Expedition, brought out precisely the same result. The 

 alkalinity of bottom waters was found to be distinctly 

 greater than that of those from the surface, and this in- 

 crease was exactly proportional to the larger quantity of 

 lime present in the former. The alkalinity determinations 

 give the only satisfactory measure of the carbonate of 

 lime which exists as such dissolved in sea water. 



The last and longest chapter is by no means the 

 least important. It deals in a very exhaustive manner 

 with the whole question of the absorption of oxygen and 

 nitrogen by pure water, as well as by sea water. Finding 

 that all previous determinations of the coefficients of ab- 

 sorption of these gases were more or less unsatisfactory, 

 Prof. Dittmar entered on an elaborate series of experiments, 

 which is fully detailed in the memoir, having as an aim 

 the determination of the desired coefficients at different 

 temperatures for both pure water and the water of the 

 ocean. The second part of the research consisted of the 

 exact analysis of the samples of air which had been ex- 

 tracted from sea water on the cruise by Mr. Buchanan, 

 using Jacobsen's ingenious apparatus, which he has since 

 simplified and improved. 



There is an element of uncertainty about the extraction 

 of gases which Prof. Dittmar scarcely seems to emphasise 

 sufficiently. The water in every case was collected in 

 one vessel and then transferred to another in which the 

 gases were boiled out. The danger of atmospheric gases 

 being absorbed was obviated in great measure by the pre- 

 cautions used ; but if highly aerated water were brought 

 up from a great depth some of the absorbed gases would 

 be certain to escape during transference. The only 

 remedy would appear to be the collection of the water 

 in the gas-extraction flask itself, and there is no method 

 as yet by which this can be done. 



Prof. Dittmar and Mr. Lennox constructed a modifica- 

 tion of Doyere's apparatus for the analysis of gases, which 

 was found to work well. It is described and figured in 

 the memoir. The results of the analyses are discussed 

 in the light of the coefficients of absorption found in the 

 earlier part of the research. The amount of air which 

 ought theoretically to be absorbed by sea water of the 

 temperature and at the pressure at which each sample 

 was collected was first calculated, and then, from the 

 actual amount of nitrogen found, the quantity of oxygen 

 which should be associated with it was arrived at. The 

 quantities of air found in solution were usually in defect 

 of calculation, as might be expected when it is recollected 

 that the water of the ocean is always in motion, the tem- 

 perature and pressure to which it is exposed being very 



