I go 



NA rukE 



{Jan. 6, 1876 



be once conceded as a result of theoretical considera- 

 tions ? 



Before briefly discussing these questions, let us refer for 

 a moment to one or two other minor points which deserve 

 a passing consideration as bearing upon the state of the 

 marine aquarium as a miniature sea, the health of whose 

 inhabitants is ensured in proportion as its actual condi- 

 tions approach to those of its natural prototype. 



It has been suggested that the proportion of carbonic 

 acid held in solution in the water is a matter of more im- 

 p>ortance than has been recognised, and that the effect of 

 the constant influx of a copious and finely comminuted 

 stream of air passing night and day through the tanks is, 

 after pretty completely oxidising the organic matter with 

 which their contents are charged, to displace the resulting 

 carbonic acid, and so reduce its percentage below the 

 normal amount present in the ocean ; and a parallel has 

 been drawn between the condition of the inhabitants 

 under these circumstances and that of human beings 

 breathing an atmosphere containing an abnormal propor- 

 tion of oxygen. 



In confirmation of this opinion, it has been pointed out 

 that one of the prominent results of the Challenger 

 researches is that animal life is abundant at the bottom of 

 the sea, while the amount of carbonic acid held in solu- 

 tion in its lowest strata exceeds that of the surface layer 

 by six or seven per cent. 



Now, it is certain that, supposing all sources of further 

 or continuous supply of carbonic acid to be excluded, the 

 exposure to the air of any given bulk of water containing 

 the maximum quantity of that gas which it is capable of 

 holding in solution (or any less quantity) will finally result 

 in its total elimination ; for Dalton long ago established 

 laws from which it follows that when water saturated with 

 a gas, e.g. carbonic acid, is placed in contact with the 

 open air, the whole of such gas is set free, while the water 

 absorbs the constituents of the air. 



Hence the small quantity of carbonic acid always 

 present in sea-water is not due to absorption from the air, 

 but to the incessantly renewed supply aff'orded by the 

 oxidation of organic matter in the sea itself. 



If this supply were not constantly maintained, this con- 

 stituent would vanish from the ocean. Its higher per- 

 centage in the lower strata of the sea is doubtless due to 

 three causes : i, to the comparative stillness of the water, 

 whereby the diffusion of the solution is retarded ; 2, to 

 the absence of direct contact with the air and exposure to 

 the wind ; 3, and chiefly, to the increased pressure, 

 whereby solution of the gas is greatly facilitated ; for 

 under pressure of i atmosphere and at ordinary tempera- 

 tures, I cubic centimetre of water dissolves in round 

 numbers i cubic centimetre, or I'Sag milligrammes, of 

 carbonic acid, while under double that pressure the absorp- 

 tion is double, and so forth, varying oirectly as the pres- 

 sure, app7-oxiniately. 



It can hardly be doubted that this presence of a larger 

 proportion of carbonic acid in the lowest depths of the 

 ocean has a distinct correlation to the character of their 

 special inhabitants. 



Prof. Wyville Thomson writes : "In the 'warm area,' 

 and wherever the bottom is covered with ooze, calcareous 

 forms predominate, and large sandy cristellarians, with 

 their sand-grains bound together by calcareous cement, 

 so that the sand-grains show out, dark and conspicuous, 

 scattered on the surface of the white shell." And again : 

 " The dredging at 2,435 fathoms at the mouth of the 

 Bay of Biscay gave a very fair idea of the condition of 

 the bottom of the sea over an enormous area. . . . The 

 surface layer was found to consist chiefly of entire shells 

 of Globigerina bulloides, large and small, and fragments 

 of such shells mixed with a quantity of amorphous cal- 

 careous matter in fine particles ; " and he proceeds to 

 trace how the gradual subsidence of this ooze is forming, 

 under the pressure of superincumbent water, vast geologi- 



cal strata, just as they have been formed for countless ages 

 in the past. 



Now, carbonate of lime is much more freely soluble in 

 water containing carbonic acid than in pure water. Hence 

 the abundant supply of this substance in the bed of the 

 ocean is doubtless freely taken up into solution, to be in 

 turn abstracted and secreted by fresh generations of living 

 animals ; and thus the carbonic acid forms, as it were, a 

 carrier or circulating medium, if not essential to, at any 

 rate vastly facilitating the ever-alternating processes of 

 life and death, by which the surface of the submarine 

 globe is being constantly and profoundly modified. 



But, on the other hand, the animals kept in aquaria are 

 essentially surface-dwellers ; the tubicolous Annelids, 

 Actiniada;, Echinoderms, Crustacea, Nudibranchs, Mol- 

 lusca, and fishes, which can be successfully kept in confine- 

 ment all belong to this category, and are captured either 

 between tide-marks or within a few fathoms of the sur- 

 face. Coral-building Anthozoa perish as the subsidence 

 of the areas in which they dwell plunges them into depths 

 exceeding fifteen or twenty fathoms, a fact which is the 

 basis of Mr. Darwin's simple and elegant theoiy of the 

 formation of circular reefs and " atolls." 



It appears that the amount of carbonic acid present in the 

 tanks of a marine aquarium must represent the balance 

 between the quantity evolved from decaying animal matter, 

 exuviae, excreta, remnants of food, and the like, and that 

 eliminated from the water by the absorption of air. It has 

 probably never been determined, and its accurate estima- 

 tion would be a problem both easy and interesting. 



But in comparing or contrasting the conditions under 

 which animals live in the confinement of the tanks with 

 those which prevail in the open sea, it must not be for- 

 gotten that there is present in an artificial collection of 

 animals an element wholly different from those which 

 exist in the ocean. 



In the upper layers of the sea, at any rate, the bulk of 

 water passing over any given animal is tens of thousands 

 of times greater than if the whole contents of the largest 

 aquarium were circulated over it daily. In comparison, 

 the animals are far more sparsely distributed, and dead 

 organisms, together with rejectamenta of all sorts, are 

 swept away by the first tide, and practically got rid of 

 oncf and for all, so far as their effect on the living indi- 

 vidual is concerned. 



In the aquarium, animals are disposed in groups arti- 

 ficially brought into comparatively close juxtaposition, and 

 direct oxidation is the only means of removing the various 

 organic impurities of which they are the source. 



The observations of the Challenger naturalists show 

 that the amount of organic matter in surface and bottom 

 waters is about the same, being about 2| times as great 

 as in intermediate strata. It corresponds, therefore, to 

 the more abundant distribution of animal life ; yet no 

 one can doubt that the percentage of organic matter pre- 

 sent in the aquarium vastly exceeds that in the sea, 

 owing to the non-dilution of decomposing animal matter 

 by any such enormous influx of pure water as is supplied 

 by the tides and currents of the ocean. 



Hence there is an evident necessity for much more 

 direct aeration in order to prevent the accumulation of 

 organic matter either dissolved or suspended. 



Pari passu with aeration, the formation of carbonic 

 acid increases, but as this substance is in turn eliminated 

 by excess of air, experiment alone can determine whether 

 the amount present in the aquarium, as nov/ worked, is 

 greater or less than that contained in either the upper or 

 lower strata of the ocean. 



We may, at any rate, safely conclude that it is of the 

 utmost importance to have command at all times of a 

 superabundant power of aeration. 



The possibility of increasing it to meet the emergency 

 of some sudden temporary pollution arising from the 

 death of some of the inhabitants, the careless introduction 



