432 



THE MICROSCOPE. 



dred-and-eighty-seven-millionth part of a grain. The 

 well-known Turkey stone, so much used for the purpose of 

 sharpening razors and tools ; the Rotten-stone of com- 

 merce, a polishing material ; and the pavement of the 

 quadrangle of the Royal Exchange, are all composed of 

 infusorial remains. 



Fig. 224. 



1, Shell of ArachnohUsrus. 2, Acti>>ocyclus (Bermuda). 3. Cocc'neis (Algoa B.iyj. 

 4, Cuscinodiscus( Bermuda.) 5, tslhmia enervis. 6. Zygocerm, rhombus. 



The bei'gh-mehl, mountain-meal, in Norway and Lapland, 

 has been found thirty feet in thickness ; in Saxony, twenty- 

 eight feet thick ; and it has also been discovered in Tus- 

 cany, Bohemia, Africa, Asia, the South Sea Islands, and 

 South America ; of this, almost the entire mass is com- 

 posed of flinty skeletons of Diatomacece. That in Tuscany 

 and Bohemia resembles pure magnesia, and consists 

 entirely of a shell called campilodiscus, about the 200th 

 of an inch in size. 



