THE DIAMOND AND OTHER PRECIOUS STONES. 343 



cut would appear to warrant this supposition. An art has been lost to 

 France. Who will restore it ? Since the last encouragements given to 

 engraving on stone by the Empress Josephine and ISfapoleon I, every- 

 thingof tills kind comes to us from Italy, and there is not a single glyptic 

 monument of the reigns which have succeeded the Empire, 



The diamond is heavier than rock-crystal and lighter than white 

 sapphire. It has almost the same weight as the white sapphire of Bra- 

 zil, called goutte (Veau. It is often confounded with these three stones, 

 which resemble it in whiteness. Let us see then how these ma}^ be dis- 

 tinguished by the weight. 



It is known that if a real diamond be suspended by a fine thread from 

 a delicate balance, and when in perfect equilibrium it be immersed in a 

 glass of water placed immediately under it, it loses two-sevenths of its 

 weight, or, in other words, two-sevenths of its weight in air must be 

 added to the pan from which it is suspended to restore the equilibrium 

 when the diamond is in the water. In like manner, a diamond weighing 

 21 carats loses in water about C centigrams. A white sapphire of 

 the same weight loses only a fourth of its weight when weighed in 

 water, that is to say, about 5 centigrams. A piece of rock-crystal in 

 the same conditions loses 8 centigrams. Hence, whenever any species 

 of crystal weighed in water loses more than two-sevenths of its actual 

 weight, it cannot be a diamond. We shall presently see how the dia- 

 mond is distinguished from the white topaz, which, like itself, loses in 

 water two-sevenths of its weight. 



Chemical tests being, in general, very difficult of application, and in- 

 volving a loss of the substances examined, need not here be described ; 

 but we shall point out an oi)tical test of a very delicate character which 

 traces at once the line of demarkation between the diamond and all 

 other colorless gems ; we refer to that of double refraction. 



In looking through a transparent stone at a detached object, such as 

 the point of a needle, or a small hole jnerced in a card, the object is seen 

 double, xas if there were two needle-points or two holes. This phe- 

 nomenon is called double refraction, aud is exhibited by all white or 

 colorless gems except the diamond. As some little dexterity is required 

 to readily exhibit this curious property, the object to be looked at and 

 the stone should be fixed at the proper distance apart on a support by 

 a little modeling-wax, so as to be more conveniently seen by those inter- 

 ested in the experiment. M. Haiiy was often called upon in consultations 

 of this kind, and sometimes, in the case of a suspected fraudulent sale, 

 he gave his testimony in court as an expert in regard to the character 

 of gems. The white topaz of Brazil produces double refraction, and 

 may at once be recognized by this quality as a false diamond. I have 

 a painful recollection of a visit from an English gentleman, who brought 

 for my examination a magnificent white topaz, which, had it been 

 diamond, would have been of immense value. It was very easy for me, 

 from the cutting of the stone, to perceive the double refraction ,• but 



