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holding silex in solution ; that silicious crystals form all round the 
cavity, 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 
superior 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 
together, 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 
