On the Assay of Minerals by means of the Blowpipe. 44) 
warious tints Which they assume on becoming red, from the 
faint red to the white. . ; 
Phosphorescence, when it takes place, presents also various 
colours, which change sometimes, as we see in the apatite and 
fluor spar. ‘We may remark also the colour assumed by the 
flame itself; the sulphate of strontian, for instance, gives to 
the blue part of the flame a reddish tinge. 
As to changes in the mineral, these may take place without 
any alteration in its form, such as the change or loss of colour. 
The surface may become deeper, either in whole or in part, as 
is the case with pyrites. 
The yellow oxide of iron becomes totally red, the flowers of 
cobalt blue, &e. There is also a change of colour, or loss of 
lustre, in white mica, lamellated gypsum, &c, There is a change 
in the refrangent property, when the molecules of a fragment 
which was diaphanous separate, from the loss of the water of 
erystallization, or the enlargement of their interstices, which fre- 
quently gives an opportunity of discovering the texture of varjous 
substances, such as ponderous spar, &c. There is also a change 
of consistence : it inereases, as in potter’s clay, and diminishes in 
others, such as lime, which the action of heat renders porous. 
Various odours and tastes are also developed by heat ; and there 
may be an alteration of form without change of substance, by 
the liquefaction of the water of crystallization, of alum, bo- 
rax, &c. 
Decrepitation.—When a mineral bursts into fragments, we 
must distinguish whether the noise be great, and the pieces large, 
er the contrary. 
Evaporation.—In this case the molecules of a substance are 
detached in form of vapour, without any alteration in their sub- 
stance; as water, mercury, &c. 
Exfoliation.—Separation of the lamine of a mineral, from 
which an increase of volume results; as in lamellated gypsum, 
apophyllite, stilbite, triphane, &c. 
Effiorescence.—Is the formation’ of smal] mossy excrescences 
on the edges of the fragment, a phenomenon probably pro- 
duced by some disengagement of gas. 
Formation of air bubbles.—-Solitary air bubbles are formed 
on the surface of the mineral, as in the pyrophysalite, probably 
by the disengagement of the fluoric gas. 
General swelling.—When the mineral increases in volume by 
the formation of a multitude of small bubbles, which give it 
the appearance of a froth ; is this to be ascribed to a disengage- 
ment of gas, or a development of vapours? It is difficult to 
say which. ‘This fact is remarkable in the meionite, the lepi- 
dolite, &c, 
Boiling. 
