190 Chemistry. [Lecture 30. 



stinguish it by its colour. The best is of a 

 beautiful deep green, and, when viewed through 

 the light, is variegated with reddish or purplish 

 spots ; it is then called blood-stone. 



To this head may be referred that beautiful 

 blue stone lapis lazuli^ the principle to which it 

 owes its colour is not decidedly ascertained. 



Crystals are always found in the cavities of 

 veins, and in the strata of flinty matter, when 

 these are not filled up with quartz. Some are 

 quite transparent, others white, and others of a 

 dark or smoky colour, which they lose in the fire ; 

 but they are very liable to be cracked during the 

 operation. Some of these crystals are more re- 

 gularly formed than salts, and are hexagonal. 

 Some are small and opaque, and have a pyramid 

 at both extremes. They are called rock crystal, 

 from the situation in which they are often found. 



Gems are found in the state of crystals, or of 

 pebbles ; the last are most valued, and are found 

 in the strata of gravel, from which they are di- 

 stinguished by their lustre. 



The gems are distinguished by a greater lustre, 

 and resistance to fire, and hardness ; but most of 

 them seem to have the same earth for their basis 

 as the rest, viz. silica. The diamond consists, 

 however, of pure carbon ; and the sapphire is 

 chiefly alumina. 



Lastly, we find the flinty matter in petrifac- 

 tions, as of wood, shells, corals, &c. 



The first effect of heat on flinty substances is 



