I 
the Corundum Stone , and its Varieties , &c. gig 
Tourmalin. This substance is also frequently found in the 
sand of Ceylon : indeed it is in this sand that the most per- 
fect crystals of tourmalin, the most transparent, and the most 
various in colour, are generally found. It is certainly to be la- 
mented, that these crystals are seldom of any considerable size ; 
but that defect is compensated by the perfection and regularity 
of their form. Among these, I have found two in particular, of 
which, as they have not hitherto been noticed, I think it right 
to give a description. 
The first of these forms, is the very obtuse rhomboid which 
Is represented in Fig. 52, and is the primitive crystal of this 
substance. The Abbe Hauy, who also thinks that this rhom- 
boid is really the primitive form of this substance, appears not 
yet to have met with it ; for he has not placed it at the head 
of the description of tourmalin given in his mineralogy, as he 
has done with respect to the other substances of which he has 
observed the primitive form. It is indeed very scarce. I have, 
however, met with it several times ; and have placed a very fine 
specimen of it in Sir John St. Aubyn's collection. This crystal, 
which is about four lines in diameter, and nearly two lines in 
thickness, is of a brown colour with a tinge of orange • it is 
also pretty transparent, even in the direction of its axis. Its 
form is perfectly well defined ; and the two pyramids, of which 
its rhomboid may be considered to be formed, are exactly similar 
to each other; neither of them having any supernumerary 
facets. 
I tnink ft right here to observe, that there appears to me to 
have been an error committed, with regard to the measures that 
have been given as those belonging to the primitive crystal of 
the tourmalin. The Abbe Hauy fixes the measure of the solid 
