252 



UTILIZATION OP MINUTE LIFE. 



Under the name of Tripoli are included several 

 of these siliceous infusorial earths, extensively em- 



u 



FIG. 32. Gallionella ferruginea. 

 1. Magnified 300 times. 2. Magnified 2000 times. 



FIG. 33. Gallionella distans. 



ployed for polishing metallic surfaces, etc. They 

 derive their name from Tripoli, in Barbary, whence 

 the substance was originally procured.* 



Is it not an interesting fact that the remains of 

 creatures individually invisible to the naked eye, 

 should, in course of time, form rocks and strata 

 destined to figure among the economical appli- 

 cations of the human race ? 



Since 1836, Ehrenberg has observed that the 

 organic forces are still so active in the mud of 

 ports and rivers, that at Swienemiinde, in the 

 Baltic, for instance, where more than two and a 

 half millions of cubic feet of mud were recently 



* Some kinds of Tripoli are entirely mineral, but these are 

 generally known as Emery. 



