[ HARRINGTON ] MINERALOGICAL CHEMISTRY 9 
hydrofluoric acid are also worthy of study by those interested in the 
question under consideration. 
The different action of solvents upon different faces of crystals has 
long been recognised. Lavizzari, for example, observed in 1865 that on 
immersing calcite crystals in an acid, faces of like kind gave off the same 
quantity of carbon dioxide, but that the gas was evolved much more 
abundantly from the lateral than from the end faces of a hexagonal 
prism.! The investigations of Baumhauer and others have also shown 
that when a crystal is submitted for a short time to the action of a solvent 
capable of attacking it, the faces are not uniformly attacked, but are 
covered with little cavities of characteristic form. The cavities, which 
have been termed corrosion or etch-figures, are generally polyhedral in 
form and similarly orientated on the same face or on faces of like kind. 
Like the optical characters of a mineral, they point to a most intimate re- 
lationship between the internal structure and external form of crystals 
and sometimes afford a valuable means of detecting unsuspected merohedry. 
The group of silicates is one affording many difficult and interesting 
problems in chemical constitution, to some of which attention may be 
called. The various members of the group have by some been regarded 
as compounds of metallic oxides with silica—enstatite, for example, as a 
compound of MgO with SiO,, just as magnesium sulphate was once 
looked upon as a compound of MgO and SO,. The view now generally 
accepted, however, is that they are salts of different silicic acids, either 
known or hypothetical. This is certainly the most rational way of look- 
ing at them, and even if we make assumptions as to one or more ‘“‘ hypo- 
thetical silicic acids,” we do nothing that is not constantly done in the 
case of carbonic acid. 
Of the silicic acids, the simplest are orthosilic and metasilicic, whose 
constitution may be represented thus : 

A (ONE A 
RQ a : 
Sie” Si OH 
OH oe 
OH Ou 
ORTHOSILICIC ACID. METASILICIC AOID. 
The latter, it may be observed, may be regarded as an anhydride 
of the first. 
There is little doubt as to the existence of orthosilicic acid, though 
it does not seem to have been obtained in a perfectly pure state. A 
number of its compounds, with organic radicals (esters), have been 
prepared, and the vapour density in some cases determined. We have, 

! Elements de Cristallographie physique. Soret. Genève, 1863, p. 222. In this 
conne*tion see also an interesting paper by Meyer and Penfield on ‘‘ Results obtained 
by etching a Sphere and Crystals of Quartz with Hydrofluoric Acid.” Trans. Conn. 
Acad., vol. viii., 1889. 
