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 Araehnoidiscus. 2, Actinocyclus (Bermuda). 3, Cocconeis (Algoa Bay). 

 4, Coscinodiscus (Bermuda.) 5, Isthmia enervis. 6. Zygoceros rhombus. 



The bergh-mekl, mountain-meal, in Nor way 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 Diatomaceas. That in Tuscany 

 and Bohemia resembles pure magnesia, and consists 

 entirely of a shell called campilodiscus, about the 200th 

 of an inch in size. 



