ing silex in solution ; that silicious crystals form all round the ca- 

 vity, except at the aperture, where there is nothing to attach them- 

 selves to ; and that an aperture in some other part allows a regular 

 escape of the fluid, by which a correspondent supply is demanded 

 from the first-mentioned aperture, until, after, perhaps, the lapse 

 of ages, the crystals fill up, from the bottom and sides, to the aper- 

 ture by which the fluid was admitted, and form a solid crystallized 

 mass, from which the water is excluded. Nor does it even appear 

 necessary for the formation of a solid crystalline mass, that there 

 should be more than one opening, that which admits the fluid into 

 the cavity ; since the calibre of the opening bearing a proper pro- 

 portion to the cavity to be filled, it will remain open until the mass 

 of crystallization is completed ; it being reasonable to suppose, 

 that the fluid containing the silex would, in consequence of its su- 

 perior gravity, be continually supplying the place of that, which, 

 having been deprived of its silex by crystallization, would of course 

 tend upwards ; and thus might the cavity become entirely filled by 

 crystallization, to the total exclusion of the solvent. The sections 

 of agatine nodules indeed show complete and uninterrupted con- 

 centric coats, without any such departure from the circular form 

 as would point out the opening by which the cavity had been filled ; 

 but then it is to be considered, that it is only in one particular 

 direction, that this could be shown, in a section ; in every other 

 direction, the concentric coats will appear perfect and regular. But 

 when the laws on which crystallization depends are considered 

 it being a process in which the suspended molecules are united to- 

 gether, by their particular attractive forces, according to certain 

 laws regulating their aggregation, and directing the forms which 

 they assume, to the exclusion of water, except so much as enters 

 into chemical union in their crystals surely it is not contrary to 

 reason to suppose, that a cavity may thus be filled by crystallization, 

 to the total exclusion of the solvent. Thus, then, I conceive, solid 



