> tse 
Parr L. Sect. iv. § 1.] EXPERIMENTS IN FUSION. 293 
crystallisation sets in, the glass is devitrified, and a lithoid product 
is the result. 
Illustrations of the influence of different degrees of heat upon 
rocks of various kinds may often be very instructively observed at 
lime-kilns, especially those roughly-built kilns or pits which may 
still be met with in outlying districts. Some of the stones lining 
such cavities will be found with no sensible change, others show a 
somewhat cellular, others a rudely prismatic structure, while some 
have had their surfaces fused into a rough glaze or enamel. ‘The . 
bricks or stones used for lining furnaces present similar illustrations. 
In these and other effects, when produced by the contact of hot 
intruded igneous rocks, the alteration is merely local, and has 
obviously been produced either by contact with a highly-heated 

surface, or through the operation of heated vapours escaping from 
the eruptive mass. But, besides such minor effects due to contact, 
others of a more general kind affect large masses of rock or whole 
districts of country (Book IV. Part VIII). 
The effect of heat in the open air upon different minerals varies 
considerably. Thus a few, such as native arsenic and calomel, pass 
into vapour without melting and form sublimates. But many re- 
fractory substances may be made to sublimate in the presence of 
other vapours, in particular, of fluorine and boron (see p. 302). 
Some minerals (sulphur, for example) pass at once, others (like mica, 
olivine, and hornblende), almost at once, from the liquid into the 
solid condition, as water does in freezing. The majority, how- 
ever, after fusion, have an intermediate viscous stage, like that 
of iron and glass. Many minerals can be made to crystallize again 
after fusion (augite, garnet, calcite, rock-salt, fluor-spar), or can be 
artificially produced by the melting together of their component 
ingredients (augite, apatite, pyromorphite); others, however, remain 
in an amorphous vitreous condition.* 
A glass is an amorphous substance resulting from fusion, 
perfectly isotropic in its action on transmitted polarized light (ante, 
pp- 99, 189). Itsspecific gravity is rather lower than that of the same 
substance in the crystallized condition. By being allowed to coal 
slowly, or being kept for some hours at a heat which softens it, glass 
assumes a dull porcelain-like aspect. This devitrification 
possesses much interest to the geologist, seeing that most volcanic 
rocks, as has been already (p. 104) described, present the characters 
of devitrified glasses. It consists in the appearance of minute 
erystallites, and other imperfect or rudimentary crystalline forms, 
accompanied with an increase of density and diminution of volume. 
It must be regarded as an intermediate stage between the perfectly 
glassy and the crystalline conditions. | 
Rocks exposed to temperatures as high as their melting-points 
fuse into glass which, in the great majority of cases, is of a bottle- 
green or black colour, the depth of the tint depending mainly on the 
? Roth, Cham. Geol. i. p. 40. 
