236 
The crystals of Albite also seemed to possess but slight power 
of cohesion, and the plates separated were thinner than those 
of the Orthoclase. 
The quartz, with the exception of cracks was fresh and not 
greatly changed, yet had an oily look. It showed by the 
coloured rings, that force had affected the elasticity of the 
mineral. In form, some of the crystals had a bulging along the 
planes o P, the course of the isotherms of the prisms. The 
quartz has a higher conductivity in the direction of the optic 
axis than in the direction perpendicular to it. In some crystals 
was an iridescence and play of green and red hues, like an opal. 
The Biotite, or magnesian mica alters more easily than potash 
mica, and had exfoliated into numerous scales of light brass 
colour. 
The examination was then advanced by submitting some of 
the burned granite to a crucial test. 
Experment 1. A portion of the charred substance was 
placed in the fire and brought to a full Cherry Red colour. It 
was then allowed to cool in the cold air; when examined, the 
cracks running through the granite had become wider and 
deeper, affecting the felspars peculiarly, with zonal margins 
encircling and bordering the crystals after the manner of that 
figured in Cohen’s Sammlung (T. xx. f. 4). The red colour of 
the Orthoclase had become as it were curdled into mottles of a 
brownish hue, being a re-arrangement of the ferric constituent 
under high temperature, as iron when in amounts of from 2 to 
5 per cent. is of importance, for it affords changes and colour 
indications when acted on by heat. The hardness of Orthoclase 
is equal to that of window glass when referred to Moh’s scale. 
The minimum of hardness is upon the middle and cleavage 
planes, the maximum is in the direction of the faces normal to 
the cleavage-planes of the crystal. 
Exereriment 2. A blackened portion, in this trial, was 
raised to a furnace heat, a Low White colour, and allowed to cool 
slowly in the cold air. On inspection the refractory quartz was 
found much affected, cracked every way; it had become some- 
what duller, the opalesence disappeared, and the black surfaces 
