The Inaugural Address. 



277 



the result of chemical changes in the solution ; and even crystallised 

 quartz has in most cases certainly had a similar origin, though its pre- 

 sence in certain igneous rocks points to its being also in some cases 

 formed where a high temperature reigned. The beautiful mineral, 

 agate, is chalcedony for the most part. Its banded structure, its 

 waved succession of parallel layers, some translucent and some 

 opaque, are too familiar to need description; but, numerous and 

 delicate as are the layers, which are the characteristic features of 

 agate, they point to the certain conclusion that each successive layer 

 has been deposited after one on which it lies. Indeed, each layer 

 has in general a character of its own, by which it differs from the 

 adjacent layer, it may be in thickness, or in colour, or in transparency, 

 or porosity. 



These latter differences are in general due to the variable pro- 

 portions in which the amorphous (or uncrystalline) kind of silica, 

 chalcedony, is mixed with the crystalline variety of the same sub- 

 stance, quartz, which is intimately associated with the chalcedony 

 in crystalline particles too minute for distinct recognition in the 

 microscope. 



Let us consider this for a moment, for it bears in an important 

 manner on our flint problem. Quartz and chalcedony, though 

 identical in composition, present very remarkable points of differ- 

 ence : chalcedony is lighter than quartz in the proportion of about 

 four to five, and it is very readily dissolved in the alkalies potash or 

 soda, or their carbonates. Quartz is not so. Let us go a little 

 further into the chemistry of these dual phases of the same substance. 

 When silica is first separated from certain of the silicates — and this 

 is easily effected by means of a stronger acid — the separated silica 

 presents itself as a hydrated silicic acid in a more or less jelly-like 

 form ; and in this condition, though very soft and gelatinous, and, 

 when dried, brittle and fragile, it can under proper conditions be 

 obtained of a hardness sufficient to scratch glass. It then has a 

 horn-like aspect, much resembling chalcedony, though in composition 

 it is more nearly allied to another mineral form of silica, containing 

 a considerable amount of combined water, namely, opal. 



But if the experiment of separating the silicic acid be performed 



