85 
Form of the Integrant Molecule of Carbonate of Lime. 
rhomboid which they traverse, and adhering firmly to the two 
surfaces between which they are interposed. Their existence is 
always indicated by striae parallel to the long diagonals of the rhom- 
boidal faces. These striae are indeed the actual edges of the veins, 
and there can be no doubt that the cleavages observed by Count 
Bournon are the faces disclosed by separating the vein from the mass 
which embraces it. In order to determine this point by direct 
experiment, I took a very transparent piece of carbonate of lime, in 
which the veins were distinctly seen, and having placed the edge of 
a knife at the junction of the vein with the mass, I succeeded, by a 
smart blow, in separating it from the contiguous surface of the rhom- 
boid. In all other places, except at the edges of the vein, the 
rhomboid resisted every attempt to cleave it. The two disunited 
faces were as perfectly polished as the finest gems, and surpassed in 
distinctness any of the common cleavages of the spar. 
That one of the two separated surfaces was the surface of a vein, 
and the other a surface formed by juxta-position to the vein, was not 
only obvious from direct examination, but was capable of being 
proved by various optical considerations. The two images, for 
example, formed by the half of the rhomboid which contained the 
vein, were actually depolarised by its action, while the two images 
formed by the other half, had suffered no change in their polarisation. 
Another proof, equally satisfactory, is derived from the determina- 
tion of the laws which regulate the action of crystallised surfaces upon 
light. By these laws I have been enabled to determine the direc- 
tion and position of the sides and angles upon any surface of a piece 
of opaque or transparent carbonate of lime, from which all ti aces of 
an external crystalline form have been obliterated. In this way the 
surface of a vein can always be distinguished from the surface con- 
tiguous to it, and by the application of this infallible test, I have 
