140 Royal Society : — Influence of Animal 



able deposit made its appearance on the leaves of the plants and the 

 glass front of the containing vessel, which was found to consist of 

 carbonate of lime in a crystalline condition. This deposit formed a 

 nidus for the growth of confervoid vegetation, which, at certain sea- 

 sons of the year, increased very rapidly. These observations were 

 alluded to at one of the Friday- evening meetings of the Royal Insti- 

 tution, March 27, 1857, when portions of the deposit were exhi- 

 bited, and its composition demonstrated by experiment. 



The formation of this deposit was then explained as arising from 

 the fact that, as the summer season advances, and we have a longer 

 continuance and also a greater intensity of the light of the sun, the 

 absorption and consequent decomposition of carbonic acid by the 

 plants is carried to a much greater extent, while the quantity of 

 carbonic acid produced by the fish remains unchanged. The solvent 

 of the carbonate of lime contained in the water being thus with- 

 drawn, a deposit slowly takes place, incrusting the sides of the 

 tank, particularly towards the light, where the confervoid growth, 

 consequent upon it, accumulates in large quantities. 



In continuing these observations, my attention was particularly 

 arrested by the steady increase of deposition, attendant upon the 

 renewed activity of the leaves, during the spring; and this deter- 

 mined me to ascertain by experiment the quantity of carbonate of 

 lime existent in the water at fixed intervals during a long period 

 of time. And inasmuch as the degrees of hardness, indicated by 

 the measures of Clark's soap- test, presented a very ready, accurate, 

 and simple means of arriving at this result, that mode of estimation 

 was adopted, care being taken to displace any uncombined carbonic 

 acid by agitating the sample with atmospheric air prior to the addi- 

 tion of the test, as directed by Dr. Clark, the indications or degrees 

 thus obtained representing the quantity of lime-salts contained in 

 an imperial gallon of the sample (70*000 grains of distilled water) 

 in terms of carbonate of lime. 



In order tliat the nature of the experiment may be more clearly 

 understood, it will perhaps be better for me, before stating the 

 results thus obtained, to describe briefly the construction and 

 arrangement of the aquarium, its position, and its contents. The 

 tank consisted of a rectangular zinc framing, twenty inches long by 

 thirteen broad, and twenty-one in depth, having slate cemented into 

 it at the bottom and sides, and being glazed at the back and front. 

 It was filled with water to the height of twelve inches, or a volume 

 equal to ten gallons, and on the slate sides were cemented, at the 

 water-line, ledges of rockwork composed of sandstone and tufaceous 

 limestone from Matlock, on which were planted a few ferns, chiefly 

 Trichomanes, for ornament. The bottom of the tank was covered, 

 for about two inches, with a mixture of sandy loam and gravel, into 

 which several plants of the Vallisneria spiralis, the vegetable mem- 

 ber of the arrangement, were inserted. Some large fragments of 

 rough rockwork, principally limestone, were also placed uj)right on 

 the bottom to break up the stiff outline of the square framing, and 

 give a pleasing effect to the eye. The animal branch of the circle 



