D1ATOMACE.E. 157 



effecting physical changes in the crust of the globe, 

 far more inconceivably numerous are the hosts of the 

 Diatomacece, and far more momentous are the opera- 

 tions they perform and the influences they exert, both 

 on the world and its inhabitants. To those of my 

 readers a very considerable class, I doubt not to 

 whom a Diatom is but a Greek compound, I may 

 be permitted briefly to explain the more obvious cha- 

 racters and attributes that distinguish this universally 

 distributed, yet recondite, tribe of organic entities. 



By the general, but not quite universal, consent of 

 microscopical science, the Diatomacece are plants, 

 each composed of a single cell, invested with a coat 

 or shell of pure silex (flint), endowed with spontaneous 

 motion, and mostly found in aggregation of many in- 

 dividuals, so attached in regular series as to form 

 chains, more or less readily separable. The endoch- 

 rome, or vegetable pulp, which in most plants is of a 

 green hue, is always in this class of a golden-brown or 

 yellow, and its particles have occasionally a sort of 

 circulating movement within the cell. 



The shell, or frustule, has a fixed form and dimen- 

 sion in each species, though these are subject to very 

 great diversity in different species. Its shape is often 

 extremely elegant ; and its glassy surface is exqui- 

 sitely sculptured into pittings or prominences, which 

 are arranged in the most elaborately varied and beau- 

 tiful symmetrical patterns. 



Perhaps the most ready mode of conceiving of these 

 creatures, by one who has never viewed them with 



