and Vegetable Life on Carbonate-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- 
here. 
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 sun’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 
inthe 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. 
