1818.] Hauy, Charactb-es ties Pierres Precieuses. 379 



when he states that each difference in colour is caused by a 

 corresponding variation in the proportion of oxygen, he affimis 

 what is totally contrary to chemical experience. In speaking of 

 refracted light, he shows how, in certain cases, it may be made 

 available as a distinctive character ; thus theessonite (cinnamon- 

 ' stone), and the variety of garnet called vermeille, when viewed 

 in the ordinary way, exhibit each the same tinge of poppy red ; 

 but if they are successively applied to the eye so as to intercept 

 the reflected rays, the colour of the former will be a pure yellow, 

 while that of the other will be orange. 



That play of light, called by the French chatoiement, and for 

 which the English language wants a name, which contributes 

 so much to the beauty of the cymophane, and of the star-stone 

 corundum, may also be produced, it appears, in the garnet. For 

 this purpose a dodecahedral crystal of garnet is to be divided by 

 two parallel sections at right angles to any six of its faces ; and 

 the hexahedral lamina, thus obtained, on being held between the 

 eye and a candle, will exhibit the flame of the candle with six 

 rays streaming from it, forming angles of 60° with each other, 

 and terminating in the angles of the hexagonal lamina. 



The characters of specific gravity and hardness, which are the 

 next in order, present nothing new. Double refraction is then 

 treated of, the precautions necessaiy in making observations 

 of this kind, and the ambiguities which attend this character in 

 particular species. The electricity of minerals is discussed at 

 some length, and a method is shown of exciting this property 

 by simple pressure. For this purpose a small prism of the 

 mineral (calcareous spar for example) is to be inserted by one 

 end into a quill, and the compound bar thus produced is to be 

 evenly suspended by a silk thread to any convenient support ; 

 the projecting end of the spar is then to be compressed for a 

 second or two by the finger and thumb applied to opposite 

 surfaces. As soon afterwards as all oscillation has ceased, a 

 piece of topaz, or of any other transparent mineral, previously 

 excited by friction, is to be brought near the extremity of the 

 spar, and will immediately repel it, as, on the contrary, a piece 

 of amber, or of sealing wax, will attract it. 



The last character described is magnetism, which, however, 

 is of no great importance, being perceptible only in the garnet, 

 the peridot, and the essonite. 



An appendix follows, giving a very brief description of carne- 

 lians, jaspers, agates, and other minerals, which, though not 

 entitled to the rank of gems, are used more or less for ornamental 

 purposes. 



Ihe volume terminates with a tabular arrangement of gems 

 and precious stones, according to their colour, of which the 

 following is an abstract. 



I. Colourless. 

 Diamond. 

 White sapphire. Var. of corundum. 



