Potash on the Siliceous Skeletons of Sponges. 297 



solving substance. In many cases the explanation is to be 

 sought in the structure of the spicule itself, the edges of its 

 component layers or component rays being more readily reached 

 from witliin than from without, as, for instance, along the 

 axial canal of a linear spicule and around the central cavity 

 of a globo-stellate. 



In the case, however, of the spicular fibre of Dactylocalyx 

 and its congeners, the internal solution is still in excess, 

 though no exfoliation of lamelUe appears in them to take 

 place. Possibly the interior, and therefore older, silica of the 

 fibre has suffered some change with age, by which its solu- 

 bility has been increased ; but in the absence of any thing 

 better than mere speculation it will be as well to wait for an 

 explanation till one is met with in the natural course of future 

 observations. 



We may next observe that the solution we have accom- 

 plished in the laboratory also occurs on the large scale in 

 nature, ex. gr. at the bottom of the sea, where deciduous 

 spicules soon become partially dissolved, with the production 

 in every detail of the characters we have just described, and 

 are afterwards silted up to exhibit these characters in a fossil 

 state. But while in caustic potash Ave have a substance which 

 will dissolve some kinds of silica with the production of a 

 definite chemical compound, viz. potassic silicate, we do not 

 know, on the other hand, of the presence of any reagent in 

 sea- water which is capable of effecting the same result ; the 

 only widely diffused solvent there, with which we are ac- 

 quainted, excepting the water itself, is carbonic acid ; and no 

 one has yet shown that this acid is capable of dissolving any 

 kind of silica, or that water alone, given time enough, is in- 

 competent to the task. That one or other of these substances 

 does in certain cases dissolve that kind of silica which is 

 soluble in caustic potash is shown, however, by the changes 

 produced by rain-water on the freshly fractured surfaces of 

 black flints ; a comparatively short exposure of these to atmo- 

 spheric agencies soon causes them to lose their black trans- 

 lucent appearance, and to become opaque and white, owing, 

 as a microscopic examination proves, to the removal of some 

 of the silica from the exposed face, so as to render it at first 

 irregularly pitted and subsequently porous. The only agent 

 to which this removal can be attributed is rain-water ; but 

 since this consists both of pure water and carbonic acid, we 

 are still unable to say whether the presence of carbonic acid is 

 a necessary condition to this solution or not. As to the 

 efficacy of one or other of these substances, however, this 

 observation leaves us in no doubt ; and since both carbonic 



