triple Sulphur et, of Lead , Antimony, and Copper, &c , 33 
First Modification* This modification consists in a decrease 
along the vertical edges of the primitive crystal, so that each of 
these edges is replaced by a plane, which is equally inclined 
upon the adjacent faces, and which consequently makes, with 
each of them, an angle of 135 0 , as in Fig. 2. Among the crystals 
I have hitherto seen of this substance, I have never yet met with 
this simple octaedral prism ; it is usually combined with some 
of the other modifications, and, in that case, the sides of the 
secondary prism are generally broader than those of the pri- 
mitive crystal, as is represented in Fig. 3. The new planes 
produced by this first modification, are usually striated, and often 
very deeply, in their vertical direction.^ 
When the new planes abovementioned have acquired sufficient 
extent to cause the vertical faces of the primitive crystal entirely 
to disappear, the result is, a rectangular tetraedral prism, per- 
fectly similar to that of the primitive crystal, excepting that, in 
all I have met with, I have constantly observed that the vertical 
faces were striated, as in Fig. 4. This kind of prism is rare in 
this substance ; I have, however, met with two or three instances 
* I commonly make use of the term modification, to express that alteration which 
takes place in the primitive crystal, whenever the crystalline laminae (which are them- 
selves only a regular collection of molecules) undergo a change with respect to the 
ratio of the arithmetical progression they admit, in their deviation from the edges or 
angles of the primitive crystal ; and I employ the word variety, to signify those diffe- 
rences in the crystals which arise only from the various proportions of the faces to each 
other, or those which are produced by the combination of two or more modifications in 
the same crystal. 
+ Although, in all these crystals, the above-mentioned striae are owing to an imper- 
fection in their crystallization, I have, in the figures, represented by strokes, those 
striae which distinguish the planes of the secondary prism of this substance. By this 
means, it becomes more easy to comprehend the manner in which the crystals differ 
from each other. 
MDCCCIV. 
F 
