282 
Mr . Watt’s Observations 
1st. This substance is easily fused into glass, whose texture 
is completely vitreous, with few air-bubbles. Its fracture undu- 
lated conchoidal. Its hardness superior to feldspar, but inferior 
to quartz. It possesses scarcely any action on the magnetic 
needle^ Its colour is black : it is nearly opaque, being translucent 
only in very thin fragments. Its specific gravity appears to be 
2 743 - 
2d. The tendency towards arrangement, in the particles of 
the fluid glass, is first developed by the formation of minute 
globules, which are generally nearly spherical, but sometimes 
elongated, and which are thickly disseminated through the mass. 
The colour of these globules is considerably lighter than that of 
the glass ; they are commonly grayish-brown, sometimes in- 
clining to chocolate brown, and, when they have been formed 
near the interior surface of the cavities in the glass, they pro- 
ject, and resemble a cluster of small seeds. Their diameter 
rarely exceeds a line, and seldom attains that size, as, in general, 
they are so near to one another, that their surfaces touch before 
they can acquire considerable magnitude. In the process of 
cooling, they adapt their form to their confined situation, fill up 
« every interstice, and finally present a homogeneous body, wholly 
unlike glass, and equally unlike the parent basalt. When the 
union of the little globules has been imperfectly effected, the 
fracture of the mass indicates its structure, by numerous minute 
“ Cannay, one of the Hebrides, described by George Dempster, Esq. in the 
“ Transactions of the Society of Antiquaries in Scotland, Vol, I.”' See Mineralogy of 
the Sduth-west part of Staffordshire, by James Keir, Esq. F. R. S. published in 
Shaw’s History of Staffordshire, Vol. I. 
Mr. Kirwan states the specific gravity of rowley rag, which he calls ferrilite, at 
2748 ; and assigns its melting point at 98° of Wedgwood’s pyrometer. 
