Dr Erewster cm the Gmeral Properties of Tabasheer. 101 
fracture^ it was instantly converted into a deep and glossy 
black. 
Mr Jardine, who had the goodness to take the specific gra- 
vity of tabasheer, found that of the opaque kind to be 2.059 
when dry, and 1.320 when wet ; and that of the transparent 
kind 2.412 when dry, and 1.396 when wet. 
Mr Macie found the specific gravity of a parcel of opaque 
and transparent tabasheer to be 2.188; and Mr Cavendish^ 
having tried the same parcel, found it to be 2.169. The mean 
of Mr Jardine’s results is 2.235, which exceeds the measures of 
Macie and Cavendish, because the opaque fragments in their 
parcels must have been more numerous than the transparent 
ones, in consequence of the rarity of the latter. 
It appears from the preceding results, that in both kinds of 
tabasheer the quantity of water imbibed exceeds in weight that 
of the tabasheer itself ; and that in the opaque kinds, the space 
occupied by the pores is to the space occupied by the tabasheer, 
as 2.307 to 1 ; while in the transparent kinds, it is 2.5656 to 
This result indicates a very remarkable degree of porosity ; and 
as it makes the pore more extensive in the transparent than in 
the opaque kind, contrary to what we should expect from their 
specific gravities, it seems to follow, that the water was not ca- 
pable of insinuating itself into all the pores of the opaque taba- 
sheer. This conclusion is rendered more probable, when we 
consider the extreme difficulty with which the oil of beech-nut 
displaces the last portions of included air ; and it affords a very 
plausible explanation of the fact, that the chalky tabasheer can- 
not be rendered transparent by the absorption of water. 
We are now prepared by the preceding observations, for in- 
vestigating the cause of the remarkable paradox exhibited by 
the transparent tabasheer, in becoming perfectly opaque and 
white, by absorbing a small quantity of water, and perfectly 
transparent when that quantity is increased. As this effect takes 
place indiscriminately with all fluids, it cannot be the result of 
any chemical action, and therefore its cause must be sought for 
in the changes which the light suffers in traversing the vacuities 
of the tabasheer. 
Let ABC, Plate III. Fig. 7. be a prism of this substance, 
and abed one of its pores highly magnified. We know that 
