248 Proceedings of Royal Society of Edinburgh. [sess. 
the laying of crops by stormy weather would be prevented. The 
result proved that no more silicic acid was secreted by plants under 
these conditions than when no such addition was made. Here we 
have apparently proof that the source of silicic acid in plants lies 
beyond any question of soluble silica present in the soil. As to the 
secretion of silicic acid by marine plants and animals, we think it 
is unnecessary to formulate any elaborate chemical theory to account 
for its absorption and secretion. There can be but little doubt that 
marine plants and animals have the power of decomposing the in- 
soluble silicate of alumina, or clay, which we have seen occurs in 
all sea-waters we have examined. The experiments we have been 
able to perform with Diatoms in the carefully-washed field clay and 
pure water, appear to indicate what takes place on such an enormous 
scale in nature, and to point to suspended clay as a true source from 
w r hich siliceous organisms derive their silica. 
In the case of the secretion of silicic acid by Sponges, we may have 
another condition of things, also capable of explanation in a somewhat 
similar way. These Sponges grow in a muddy soil, and are provided 
with spicules, fixing them firmly in the deposit where decomposing 
organic matter is abundant, under the influence of which alkaline 
sulphides are continually being formed (by the deoxidation of the 
alkaline sulphates of sea-water). These sulphides may, acting 
locally, decompose the clay or silicate of alumina, setting free soluble 
silicic acid to be absorbed and stored up by the Sponges. It is not 
impossible that Diatoms, floating as they do near the surface of the 
water, may also receive silica in this manner, the organic matter 
present in the floating clay indirectly causing solution of silicic acid. 
The presence of alumina in a quantity of Diatoms obtained in the 
Antarctic Ocean seems to point not only to the original presence 
of clay in the water, but its subsequent decomposition by these 
algae. 
There is also distinct solution of silicic acid when muds consisting 
of the remains of calcareous and siliceous organisms are acted upon 
by sea-water, as shown by the following experiments. A portion of 
mixed Diatom and Globigerina Oozes was placed in a litre of sea- 
water and some mussel flesh added, so as to obtain the conditions 
attending decomposing organic matter on an ocean floor consisting 
of these mixed muds. After a week’s exposure, during which time 
