Compound Suphuretfrom Huel Boys. 6 1 
which is represented in the annexed Plate under its two prin- 
cipal appearances ; that is, having the primitive faces, the pre- 
dominant ones of the prism ; and having the secondary ones 
such, and which will be fully sufficient to make it known. In 
the first infancy of the study of crystals, it might be necessary 
to attend to every, the most trifling, variation of them, to 
trace each of their changes, step by step, to, as it were, spell 
the subject ; but in the state to which the science has now 
attained, to continue to do so would be not only superfluous, 
but most truly puerile. 
I have a very small, but very regular, crystal of the form 
of Fig. 1. 
By mensuration the faces a and m appear to form together 
an angle of about 1 35 0 , and the faces c and b an angle of about 
125 0 . 
It is said in the account above quoted, that the primitive 
form of this matter is a rectangular tetraedral prism, but no 
proofs of this have been offered ; nor have the dimensions of 
this prism been given, a circumstance of the first moment to 
the determination of true or primitive form, nor have any 
quantities been assigned to the decrements supposed. I will, 
therefore, supply these very important omissions. 
That the atom of this substance is a rectangular tetraedral 
prism, is inferable, not from the striae on the crystals, for 
striae are by no means invariably indicative of a decrement in 
the direction of them ; but from the angles which the faces a 
and c make with the faces m and b, and these angles also 
prove, that the height of this prism is equal to the side of its 
base, that is, that it is a cube. 
Hence the face a is produced by a decrease of one row of 
