Flints of the Upper or White Chalk. 185 



stances, a truly crystalline form of silica — silica being of 

 course the principal but not the sole constituent of the black 

 flint met with in the chalk, which is a compound substance 

 consisting of a purely flinty matrix, within which varying 

 numbers of the disintegrated remains of minute calcareous 

 and siliceous organisms may almost invariably be detected on 

 careful examination. Thus, in Phillips's ' Elementary Intro- 

 duction to Mineralogy,' the following, according to Klaproth, 

 are the constituents of flint — " 98 per cent, of silica, with 

 minute proportions of oxide of iron, lime, alumina, and 

 water." 



It may also be here stated with advantage that, according 

 to Graham, silicic acid or silica becomes more and more 

 insoluble the purer or more free from combined water it be- 

 comes. Hence the gelatinous compound formed by the ready 

 and intimate combination of organic silica with, and also 

 within, a mass of protoplasm, which is already an insoluble 

 colloidj is to all intents and purposes no longer either soluble 

 or miscible with loater , whilst on the question of crystal- 

 lization Mr. Graham says, " I may add that no solution, 

 weak or strong, of silicic acid in water has show?i ant/ dispo- 

 sition to deposit crystals, but ALWAYS appears, on drying, as a 

 colloidal glassy hyalite. The formation of quartz crystals at 

 a low temperature, of so frequent occurrence in nature, re- 

 mains a mystery " (Graham, loc. cit. p. 335). 



It is of the utmost importance to bear these last-named 

 characteristics of silica constantly in mind, as upon them de- 

 pends the preeminent tendency of this substance to enter into 

 colloidal combination with any albuminoid substance, such a3 

 animal protoplasm. On the other hand, it is equally impor- 

 tant to bear in mind that silica, the moment it has assumed its 

 gelatinous state, although holding in combination a certain 

 portion of water, is practically insoluble in water. Hence its 

 inherent tendency, when combined directly with protoplasm, 

 not to imbibe more water, but to part with all but the infini- 

 tesimally minute trace that remains in combination with it up 

 to the period when it is exposed to atmospheric agencies on 

 dry land — this expulsion of its combined water being the 

 result partly of dialytic action, as already mentioned, and 

 partly of its idiosyncratic tendency to contract (Graham, loc. 

 cit. p. 336) more and more upon itself, and thus favour the 

 expulsion of all but the last residuary quantity, before final 

 and complete consolidation into flint ■^. This consolidation is 



♦ I have seen an interesting fact stated (but where, I am at this mo- 

 ment unable to remember), that flint-workers always find the flint softer 

 and more easy to cut away in flakes immediately after it is extracted 



