TRANSACTIONS OF SECTION D. 747 



certain depths are reached Pteropod shells are dissolved and disappear from the sea- 

 bottom, and at certain further deptbs Globigerina shells suffer the same fate, 

 Professor Dittmar holds the opinion that the solution is not due to the presence of 

 free acid, but to the solvent action of the sea-water itself, which will, even when 

 alkaline, take up additional carbonate of lime if sufficient time be given. Thus 

 the amount of carbonic acid normally present throughout the ocean cannot be 

 inimical to life ; but, according to the Professor, there must be in the depths of the 

 ocean numerous bodies of richly carbonated water, for he regards the principal 

 supply of carbonic acid to the sea-water as derived from volcanic springs and 

 discharges issuing from the ocean-bed, the quantity derived from the decay of 

 marine plants and animals being insignificant in comparison with this. Possibly 

 the ' Challenger,' when it dredged from deep water off the Azores immense 

 quantities of dead and blackened coral, encountered an area which had thus been 

 visited by a carbonic acid discharge. 



With regard to the absorbed oxygen and nitrogen, the theoretical maximum 

 quantity of oxygen absorbed at normal surface-pressure by a litre of sea-water 

 should range, according to Professor Dittmar's experiments and calculations, from 

 8T8 c.c. in cold regions at 0° C. to 4 - 50 c.c. in the tropics, with a temperature of 

 30° C. The results experimentally obtained from samples of surface-water collected 

 during the voyage differ considerably in detail from the calculated estimates from 

 various causes explained, and especially because of the reduction of the amoumt of 

 oxygen by oxidation and respiration. The main and almost sole source of the nitro- 

 gen and oxygen present in deep-sea water lies in the atmosphere and is absorbed at 

 the surface, its quantity being thus dependent on surface conditions of temperature 

 and pressure and not those of the depths. A given quantity of water, having absorbed 

 its oxygen and nitrogen at the surface, may be supposed to sink unmixed with sur- 

 rounding water to the depths. During the process its amount of contained nitrogen 

 remains constant, whilst its oxygen-supply becomes gradually diminished, owing to 

 the process of oxidation, which in the depths go on without compensation. That 

 the amount of absorbed oxygen present in sea-water diminishes with the depth 

 had been shown already by Mr. Lant Carpenter's experiments. It is not yet 

 possible to formulate in any precise terms the relation between the depth and the 

 diminution of the oxygen present, but Mr. J. Y. Buchanan's previous conclusion 

 that a minimum of oxygen is attained at a depth of about 800 fathoms is not con- 

 firmed by the summing up of the whole of the evidence now available. This result 

 is not without biological significance, since the existence of this supposed zone with 

 a minimum of oxygen has been used as an argument in favour of the occurrence of 

 especially abundant life at this depth below the ocean-surface. 



Professor Dittmar finds that there is nothing characteristic of bottom-waters as 

 such in regard to their absorbed ga?es, nothing to distinguish them from waters 

 from intermediate depths. This, it seems to me, is not quite what might have been 

 expected, as the concentration of the food supply, and consequently of life, on the 

 actual bottom might have led to a different result. 



If there were absolute stagnation of the water at great depths the oxygen might 

 be reduced there to zero, but the fact that in no case has oxygen been entirely 

 absent from any sample of deep-sea water examined proves that a certain motion 

 and change must occur. The smallest amount of oxygen found at all was in a 

 sample of water from a depth of 2,875 fathoms, and amounted to 0-65 c.c. per litre 

 only, a result long ago published by Mr. Buchanan. Even this, however, may well 

 be sufficient to support life, since Humboldt and Provencal 1 found that certain fish 

 could breathe in water containing only one-third of that quantity of oxygen per 

 litre. In another sample, from 1,645 fathoms, it was 2-04 c.c. On the other hand, 

 as much as 4-055 c.c. was found in a sample from 4,575 fathoms, and 4-39 c.c. in 

 one from 3,025. Most remarkable, in one instance water from a depth of only 300 

 fathoms yielded only 1*65 c.c. of oxygen. Professor Dittmar admits that there was 

 no lack of anomalous results, some, no doubt, due to some extent to imperfection in 

 the apparatus employed in collecting the water. 



1 ' Sur la Respiration des Poissons,' Jovrn. de Physique, de Ckimie et d'Histoire 

 tfaturelle, t. lxix., October 1809, p. 268. 



