148 THE WORLDS LUMBER ROOM. 



Silica is not enumerated among the minerals contained 

 in sea-water, because in 1,000 grains the quantity is so 

 small as to be little more than a trace. Silver, gold, and 

 many other metals and minerals, are omitted for the same 

 reason. But if there be but a trace in 1,000 grains the 

 amount dissolved in the whole of the ocean will be far from 

 trifling. It has been calculated, for instance, that the ocean 

 must contain some two hundred million tons of silver. 



With regard to the silica, we know that it exists dis- 

 solved in all water more or less, and is, therefore, conveyed 

 to the ocean in large quantities ; but, like the lime, it is 

 wanted by such innumerable living things that little remains 

 in the water. 



Silica also is greatly attracted by dead and decaying 

 substances, and collects round dead sponges, bits of coral, 

 shells, &c., and sea-urchins are frequently found filled with 

 flint, as well as embedded in it. 



So many flints have more or less the shape of sponges 

 that it seems probable these formed the nucleus round 

 which the silica collected ; and, indeed, when a thin slice 

 of flint is magnified, it is very often found to contain 

 various minute organisms, such as sponges are known to 

 feed upon. 



Toilet sponges consist of fine elastic fibres, resembling 

 silk in composition, and closely woven together. But 

 these form only the skeleton, and when alive were covered 

 within and without with a film of jelly, like white of egg, 

 but consisting of numerous individual animals or living 

 cells, which were as perpetually changing their shape as 

 a drop of liquid kept in constant motion. 



