WHAT IS PHYSICAL LIFE 



others in squares, others in triangles, 

 others in spindles or veritable microscopic 

 canoes, many of them with most beautiful 

 colors, and all with delicate lines coursing 

 over them. As the powers of great tele- 

 scopes have been rated according to their 

 resolving certain nebulas into distinct stars, 

 so the powers of microscopes have been 

 judged according to their ability to re- 

 solve the lines on certain diatoms into 

 linear dots, as in a copperplate engraving. 

 But whole strata of rocks have been found 

 in different parts of the world made up 

 chiefly of the skeletons of diatoms. One 

 such near Richmond, Virginia, is of much 

 commercial importance for making pol- 

 ishing powders, and is even used in the 

 manufacture of dynamite. But I have 

 found in the sediment of a pool in our 

 Central Park a number of the same species 

 of diatoms which are taken out of rocks 

 of the Cretaceous Period! And these liv- 

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