CHANGE OF COLOUK IN CLOTHES. 341 



others I have found to be lined with a powdery 

 matter. This last class of cavities occurred in 

 topaz, and they were distinguished from all others 

 by the extraordinary beauty and symmetry of their 

 form. One of these cavities represented a finely 

 ornamented sceptre, and, what is still more sin- 

 gular, the different parts of which it is composed 

 lay in different planes. 



When the gem which contains the highly ex- 

 pansive fluid is strong, and the cavity not near the 

 surface, heat may be applied to it without danger; 

 but in the course of my experiments on this sub- 

 ject, the mineral has often burst with a tremen- 

 dous explosion, and in one case wounded me on 

 the brow. An accident of the same kind occurred 

 to a gentleman who put a crystal into his mouth 

 for the purpose of expanding the fluid. The 

 specimen burst with great force and cut his mouth, 

 and the fluid which was discharged from the 

 cavity had a very disagreeable taste. 



In the gems which are peculiarly appropriated 

 for female ornaments, cavities containing the ex- 

 pansive fluid frequently occur, and if these 

 cavities should happen to be very near the sur- 

 face or the edge of the stone, the fever heat of 

 the body might be sufficient to burst them with 

 an alarming and even dangerous explosion. I 

 have never heard of any such accident having 

 occurred ; but if it has, or if it ever shall occur, 

 and if its naturally marvellous character shall be 

 heightened by any calamitous results, the pheno- 

 mena described in the preceding pages will strip 

 it of its wonder. 



There are no facts in chemistry more interest- 

 ing than those which relate to the changes of 

 colour, which are produced by the mixture of 



