and Vegetable Life on Carhonate-of-Lime Waters. 149 



2. The Fish. 



It is well known that water has the property of absorbing air from 

 the surrounding atmosphere, and holding it in solution to the extent 

 of from one-fortieth to one- thirtieth of its volume, not, however, 

 without somewhat changing the proportion of its constituents ; for 

 when the absorbed air is abstracted from water it usually contains 

 about thirty-two per cent, of oxygen gas, instead of twenty-one. 

 This oxygen is converted by the respiration of the fish into carbonic 

 acid, which is held dissolved by a still stronger affinity, the water 

 being capable of retaining as much as its own volume of this gas 

 in solution at the ordinary temperature and pressure of the atmos- 

 phere. 



In the above- described arrangement the carbonic acid thus pro- 

 duced is absorbed by the submerged vegetation under the influence 

 of the sun's light ; the carbon is appropriated for its growth, while 

 the oxygen is again liberated and held in solution by the water, 

 provided the evolution is not too rapid, an effect produced by too 

 great an exposure to the sun's light. When this is the case, much 

 of the oxygen necessarily escapes into the air in a gaseous state 

 and is lost. During the winter season, however, when the active 

 functions of vegetation are to a great extent dormant, from the 

 diminished quantity and intensity of the sua's light, the amount 

 of carbonic acid produced by the respiration of the fish is greater 

 than the plants are capable of consuming, and the excess must neces- 

 sarily accumulate in the water. Were the production of carbonic 

 acid confined to a short period, the water would doubtless right 

 itself after a time, the poisonous gas passing away and fresh atmo- 

 spheric air being absorbed. As, however, the production of carbonic 

 acid is constant, this ameliorating action can have little effect ; the 

 water must remain always highly charged with carbonic acid. Here, 

 then, its solvent action on the carbonate of lime, present in the rock- 

 work and gravel, comes into play, and the hardness of the water is 

 gradually increased in proportion as the light diminishes. Now, 

 supposing for an instant that no carbonate of lime had been present 

 in the arrangement, the question arises, what must then have ensued? 

 The fish would have continued to respire, and would produce carbonic 

 acid as before, which, remaining in a free state dissolved in the water, 

 would unquestionably have had a most detrimental effect upon their 

 health. Every one must have noticed the manner in which the golden 

 carp confined in a globe of water, in which there is no growing 

 vegetation to decompose the carbonic acid generated, or no limestone 

 to combine with it, rise to the surface and continually gulp in the 

 air required for their vital functions. Nothing whatever of this 

 kind has ever been noticed in the aquarium under consideration, 

 although the quantity of carbonic acid dissolved in the water has 

 been at times very large. 



From the experiments of Bischof *, we glean that the carbonic 



* Bischof's 'Elements of Chemical Geology,' Cavendish Society's edition, 

 vol. iii. p. 5. 



