298 Mr. W. J. Sollas on the Action of Caustic 



acid and water occur together at the bottom of the sea, we 

 have there just the conditions under which the solution of 

 silica must necessarily take place ; and when, in the course of 

 some years, this solution has been accomplished, it will have 

 produced just those changes in deciduous spicules which we 

 have succeeded in bringing about in a few minutes*. 



Summary. 



1. By boiling with caustic potash, spicular silica passes 

 into solution. 



2. In dissolving sponge-spicules, solution takes place both 

 on the outside of the spicule and internally about the sides 

 of its axial canal. 



3. In vitreo-hexactinellid fibre, the internal solution at first 

 reproduces as hollow casts the forms of the spicules upon 



* If for the sake of hypothesis we assume, according to all analogy, 

 that carbonic acid is the essential ingredient in atmospheric or sea-water 

 for the solution of dissolvable silica, and that some loose compound of 

 silica and carbonic acid is formed by its action, we may then be able to 

 formulate in an intelligible manner the replacement of spicular silica by 

 carbonate of lime, a change of which a large number of examples are now 

 known to spongologists. Given, for instance, a solution of calcic dihydric 

 carbonate (bicarbonate of lime) containing but a very small excess of free 

 carbonic acid, such as might naturally be expected to exist m a bed 

 composed of fragments of calcic carbonate {e. g. in a bed of chalk), and its 

 action on spicular silica might then be represented as follows : — 



(i) Dissolvable silica + (CO),(OH)2(CaO,) 



(Sponge-spicules.) Calcic dihydric carbonate. 



= Compound of silica and carbonic acid + CO(CaOJ 



(Dissolved silica.) Calcic carbonate. 



Thus calcite would become pseudomorphic after siliceous spicules, and 

 silica would pass into solution. But we also know from observation that 

 the reverse solution frequently takes place ; this we may represent in the 

 following equation : — 



(ii) C0(Ca02) + Compound of silica and carbonic acid 



Calcic carbonate, ex. gr. coral. (Dissolved silica.) 



= (CO),(OH),(CaO,) + Silica deposited. 



Calcic dihydric carbonate. 



Hence these reactions belong to that numerous class known to chemists 

 as reversible ; and it follows that the dissolved silica set free by the first 

 reaction (i) might be again deposited on coming into contact with the 

 carbonate of lime in excess in the surrounding bed, according to the re- 

 action given in equation (ji) ; and thus, in one and the same sponge- 

 skeleton, the solution of spicular silica, its replacement by carbonate of 

 lime, and the redeposition of the dissolved silica might be proceeding 

 simultaneously. In some such manner may the mineral arrangements 

 which I have described in Stmcronema and Pharetrospongia have been 

 brought about. 



