THE TECHNOLOGIST. [June 1, 1864. 



498 ON GKANITE AND ITS USES. 



hedron) of diamond, or the cube of rock-salt, has not merely a peculiar 

 external configuration, but an equally peculiar internal structure. That 

 such is the case may be easily proved. 



One of the most familiar and interesting crystals is a rhomb of Ice- 

 land or calcareous spar, one of the forms of crystallised carbonate of 

 lime or chalk. It has a highly characteristic shape, distinguished by 

 crystallographers as rhomboidal or rhombohedral, i.e., rhomb-faced or 

 lozenge-faced, because all its flat sides or faces, which are six in number, 

 have the outline of the heraldic lozenge, or the diamond on a playing 

 card. These six lozenge faces are bounded by sharply-defined, unvary- 

 ing angles. 



But the spar crystal is something more than a mere shape. A plaster 

 bust has externally the configuration of a human head, but the resem- 

 blance ceases within a line of the surface. You could not, by examining 

 a portion of the powder scraped from the back of the bust or the bottom 

 of the pedestal, tell that it had been a bust, or what it had been ; and 

 if the bust be broken by, for example, a fall, the fragments have nothing 

 in structure in common with the mere surface. So, also, an apple or a 

 rose, modelled in wax, or a grape blown in glass, may completely 

 deceive the eye, and, so far as form and colour are concerned, be mistaken 

 for a real fruit or flower, but the resemblance ends with the outside. 



On the other hand, if a crystal of calcareous spar be broken by a 

 fall or a tap with a hammer, it breaks into rhombs, each with its six 

 lozenge fa:es a perfect miniature copy of the original crystal, and those 

 small rhombs may be broken by a gentle force into smaller rhombs, and 

 these again into smaller, till the fragments, without varying their shape, 

 become microscopically small. The Iceland spar thus resembles a house 

 built of very small bricks all alike, or a piece of colourless mosaic 

 where all the portions are identical, or a piece of marquetry or Tun- 

 bridge wood-ware, consisting of exactly similar squares. 



Further, Iceland spar crystal has long been famous for its exhi- 

 bition of the double refraction of light, i. e., for its power to split into 

 two a ray of light which falls upon it in any but a single direction, so 

 as to double the image of every object seen through it ; and if one of 

 these doubly refracted and, in consequence, polarised rays be sent 

 through such a crystal in any direction but one, it produces a curious 

 array of black and white crosses and of beautifully coloured rings. 



Now, the crystal thus endowed may have its salient angles knocked 

 away, and any external configuration given to it without depriving it of 

 those endowments. It may be carved into a square block or turned in 

 a lathe into a sphere, and still it will break into lozenge-faced rhombs ; 

 Btill it will refract light doubly ; still it will show crosses and prismatic 

 rings when illuminated by a doubly refracted or polarised ray of light ; 

 still it will conduct heat, according to the law that regulates conduction 

 of heat by the unmutilated crystal ; still it will obey a magnet 

 (diamagnetically) as if it were a perfect rhomboid. 



