12 MINUTE MARVELS OF NATURE 



valves, resist putrefaction almost indefinitely ; and 

 as the organisms multiply very rapidly by sub- 

 division, and keep falling in a steady shower to the 

 bottom of the water as they perish, layers of these 



Fig. 5. Some diatoms (Pinnularia nobilis) shown 

 on a dark ground to illustrate their glassy nature. 

 Individuals vary in length from -$ to T Jjj of an inch 



glass-like shells gradually result in considerable 

 deposits of solid flinty material which resists acids 

 and even the dull red heat of an ordinary fire. 



The familiar polishing material "rotten-stone" 

 or " Tripoli," large deposits of which occur in 

 Bohemia and other parts of the world, is a soft 

 friable rock which chemically is almost pure silica; 

 but the microscope reveals the fact that it is not 



