286 DESCRIPTION OF THE MUSEUM. 



tude of rare and precious objects. The compact 

 variety, in grey masses, is common atMontmartre. 

 The sulphate of magnesia is employed in medi- 

 cine as a purgative ; it has been known by 

 the names of bitter salts, Epsom salts, Sedlitz 

 salts, etc. The borate of magnesia is of no use 

 in the arts ; but its crystals are remarkable for 

 acquiring electric properties, when submitted to 

 the action of fire. 



In the fifteenth case is the aluminous Jluate of 

 sileXy or the substance called topaz : it furnishes 

 several precious stones for jewellery ; but, how- 

 ever, it must not be confounded with that called 

 by lapidaries oriental topaz, which belongs to a 

 species of the second class. This want of agree- 

 ment between the scientific nomenclature, and 

 that in use among artists, proceeds from the dis- 

 position of the latter to unite all substances, re- 

 sembling each other in colour, under one deno- 

 mination ; although this similarity of appearance 

 frequently only disguises essential differences in 

 composition : besides which, the same species is 

 frequently subject to a series of the most diver- 

 sified tints. We have already observed such a 

 variation of colour in the modifications of fluor- 

 spar ; the topaz furnishes us with another example 

 no less remarkable. In fact, we see on the 

 shelf, for study, several specimens of a yellow 



