
PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 227 
Arran, gave the exceptionally low gravity of 2°3 ;—the weight of the crystal 
was 163°3 grains in air; 92°3 in water : 
3° Sel yi, SU 
se 2 Oe ees geen = 
It was boiled, when it gave out much air, in lines transverse to the face c; 
it was now cooled under water, dried with blotting-paper, and re-weighed 
in air, and in water. It now weighed in air 169-5 grains, and in water 98°3 
grains,—the buoyant effect of the air previously in the pores being done away 
with. 
163°3 — 98°3 = 65: oe = 2°512is therefore the true specific gravity of the 
solid matter of this crystal. 
But the weight of the crystal in air with its pores filled with air, was 163°3 
erains ; and with its pores filled with water, was 169°5 grains; the weight of 
a bulk of water = its pores is therefore 6°2 grains; and the substance 
having a specific gravity of 2°512, the bulk of its pores in its own material 
would weigh 6:2 x 2°512=15'57 grains. This, added to its original weight, 
so as to get the weight which the crystal would have been if solid, gives 
163°3 + 15°57 =178°87 grains ;—the weight of the bulk of the pores was, 15°57, 
—and 178°87 + 15°57 =114°78. 
F 1 
So that these pores are about one-eleventh and a half, 7,~z, of the total bulk 
of the solid. 
The relation between these pores in Murchisonite and the subject under 
consideration is now to be shown. 
Murchisonite is chemically an orthoclase, but it is characterised by an 
_ extra cleavage—not seen, so far as I know, in Scotch specimens,—and by a 
peculiar pearly glimmer, when viewed in certain positions. An examination 
into the cause of this optical peculiarity seemed to show that it was due to a 
structure identical with that above described as occurring in the amazonstone, 
—only that vacuities took the place of the material constituting the fibrous 
net-work. 
The crystal I was examining, however, being a complex twin of eight indi- 
viduals, was ill adapted for the display of internal structure ; and I was fortu- 
hate in having in my. hands a simple and well-developed crystal which Mr 
Dupceon had sent me to figure. 
I was not, of course, able to get a section of it, but an examination by the 
lens at once and unmistakably showed that the characteristic lustre was due to 
internal reflection from a multitude of flattened microscopic pores, arranged 
accordant with the axis of the crystal, parallel to the face a. 
