On Crystallography, 355^ 



for inspection by cutting. Those which are called gems, 

 ^and which have come through the hands of the artist, 

 tliereby becom<; susceptibht of presenting the effect of 

 ■siniple or double refraction, when we know how to guard 

 against the illusion produced by the multiplicity of tiie fa- 

 cets; and it is even an advantage to be able by ttie help of 

 this multiplicity to vary the refrangent angle, because, if 

 any one of the facets was situated in the direction of the 

 Jimits where the two refactions are reduced to one only, 

 other facets would present themselves in order to dispel the 

 doubt. We shall thus avoid confounding a piece of cry- 

 stal of Madagascar with the gem called by lapidaries while 

 sapphire, their Brazilian ruby with the lalass ruby, the 

 topaz of Saxony with what is called oriental topaz ; the 

 first stone of each of the above pairs having the double re- 

 fraction, while the second has the simple : it is fortunate 

 to be able in these cases to make up for the disappearance 

 of the crystalline forms by a physical observation, and to 

 read in a manner into the interior of the stone,when its ex- 

 terior speaks no longer to the eye. 



13. Phosplwrescenoe hy the action of Jive. In order to 

 observe this character, we must throw on red-hot char- 

 coal a small quantity of the dust of the mineral we wish 

 to examine. The phosphorescence in question is not sim- 

 ply a scintiJlation, like what is produced by the sawings of 

 wood thrown upon the flame, but a mild and agreeable 

 light similar to that of the glow-worm, the tone of colour 

 excepted, whie.h varies in different substances. This ex- 

 periment scarcely ever succeeds except in the dark. We 

 must also take great care to pound the mineral well, lest 

 any decrepitation might throw the fragments into the eyes 

 of the by-standers, 



14. Electricity, There are three ways of exciting the 

 electrical virtue in bodies ; namely, by friction, by commu- 

 nication with a body already in a state of electricity, or by 

 heat. This last method takes place only with respect to 

 certain mineral substances. 



We distiniruish two kinds of electricity : the one which 

 we call vitreous^ ajid which Franklin called positive, is that 

 which friction produces in glass ai^ other viireous sub- 

 stances. The second, which we call rennous, and which 

 Franklin described by the name of negative, is that which 

 is acquired in the same case by resin, sulphur, silk, &:c. 



These two electricities exercise contrary actions ; so that 

 two bodies, both of which are solicited by vitreous electri- 

 city or by resinous electricity, are repelled, whilst two 



Z 4 bodies 



