292 DESCRIPTION OF THE MUSEUM. 



according to its different tints. After the colour- 

 less hyaline quartz, on the stage for study, we 

 observe the violet rock-crystal, commonly called 

 amethyst; the rose-coloured or Bohemian ruby; 

 the blue, the yellow, or Indian topaz ; the yellow- 

 brown or smoked topaz ; the dark green, the 

 hcematoid of a dull red, or compostella hyacinth. 



The iridescent rock-crystal, which we see in 

 the nineteenth case, owes its name to the colours 

 of the rainbow, which are reflected by the air 

 lodged in a flaw. 



As we proceed, we remark some precious 

 specimens of all the preceding varieties on the 

 different shelves; amongst others, some masses 

 of rock-crystals from Madagascar, some beau- 

 tiful samples of rose-coloured lamellar quartz 

 from Siberia, a crystal of a blackish brown 

 hyaline quartz, from the same country, nearly 

 12 inches long. In the bottom of the eighteenth 

 case is a magnificent geode of a violet hyaline 

 quartz, given by M. Brard, formerly assistant 

 naturalist to the Museum. The variety named 

 aero-hydre, which is in the following case, con- 

 tains a drop of water, which but partly fills a 

 tubular cavity, so that the air bubble, which oc- 

 cupies the void space, ascends and descends ac- 

 cording to the motion given to the stone, as in 

 the water-level. 



