OF CALCAREOUS-SPAR. 169 



veins or interrupting strata, one of which, like abed, Fig. 7. is 

 parallel to BF, while the other sets are parallel to BC or BA, 

 the common sections of the three surfaces which contain the so- 

 lid angle. When the light is incident almost perpendicularly 

 upon one of these specimens, so as to pass through two of the 

 veins, nine images are formed ; and there is a particular posi- 

 tion of the incident ray, when all these images vanish, except 

 the colourless image in the centre. This effect can be imi- 

 tated by the transverse combination of two specimens that have 

 only one vein or interrupting stratum. 



There is another phenomenon exhibited by this interposed 

 film, which throws new light on the cohesion of solid bodies. 

 The two surfaces of the film, though in physical contact with 

 the surrounding mass, so as to adhere to it with the greatest 

 force, are nevertheless not in optical contact with it. There is 

 a distinct reflection of light at the touching surfaces, and the 

 distance of these surfaces is demonstrably greater than t,-ow!o"o~o 

 or 2 -o.l oir of an inch *. 



Now, since the surfaces of the interposed film are in every 

 case kept at such a distance from those of the adjacent mass, 

 and cannot be brought nearer to it by the force of screws, we 

 are entitled to conclude, that the particles of calcareous-spar 

 are not capable of coming into perfect contact, unless when 

 they are arranged symmetrically ; that is, when the axes of all 

 the elementary rhomboids are parallel. When these axes are 

 inclined to each other, the attractive forces by which the par- 

 ticles are held together, are so much weakened, that they are 

 incapable of forming a mass perfectly continuous, and devoid 

 of all internal reflections. This fact is the only unequivocal 

 indication with which we are acquainted, of the existence of 

 a polarity in the particles of crystallised bodies. 



* See Phil. Trans. 1816, p. 72, 73. where some analogous experiments are de- 

 scribed. 



Vol. VIII. P. I. Y X. 



