146 ftoyal 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 that 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 upright 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 
