160 M. Necker on the Mineralogical Nature of 



takes to explain the latter phenomenon, by supposing certain 

 bodies, capable at once of decomposing water, and of com- 

 bining with its oxygen without eliminating a volatile product, 

 to exist in the spots at which the action originates. 



Whatever difficulty there may be in imagining the earthy 

 and alkaline metalloids to exist amongst the number of these 

 bodies, that difficulty will at least not be enhanced by suppos- 

 ing them to be as generally distributed as the phenomena 

 themselves appear to warrant us in supposing. If it be once 

 granted, that potassium and sodium contribute, by their com- 

 bustion, to feed the fires that glow beneath Vesuvius or Etna, 

 there seems no reason why we should refuse to believe that 

 the same substances may be present, wherever thermal waters 

 or earthquakes indicate a similar train of phenomena. 



With these few remarks I am content to close the discus- 

 sion, and shall not be tempted to resume it, until it shall 

 either be shewn that the chemical phenomena observed during 

 the several phases of volcanic action are inconsistent with the 

 conditions of the theory I have adopted, or else that they can 

 be satisfactorily deduced from the supposed existence of a 

 high temperature in the interior of the globe. 



OxFORD,/i<Ke 4.1839. 



On the Mineralogical Nature of Terrestrial, Fluviatile, 

 and Marine Shells. By M. L. A. Necker. 



Brewster has remarked that mother-of-pearl, like ar- 

 ragonite, possesses two axes of double refraction, (Biblio- 

 theque Universelle cle Geneve, vol. ii. p. 182, March 1836.) Sub- 

 sequent observations, by shewing that several speKiies of ter- 

 restrial and aquatic shells are otherwise connected with ar- 

 ragonite, prove that this substance, and not calcareous spar, 

 is the matter of which almost all shells are composed. 



On examining, with the magnifying glass, a Limacella, that is 

 to say, the interior shell of a grey and black Limax,* I found 

 that the large mass of translucent, colourless, calcareous mat- 



* Limax maximus. 



