Ch. HL] 



infusoria of Tripoli. 



25 



alluded to has long been well known in the arts, being used in the form 

 of powder for polishing stones and metals. It has been procured, among 

 other places, from Bilin, in Bohemia, where a single stratum, extending 

 over a wide area, is no less than 14 feet thick. This stone, when exam- 

 ined with a powerful microscope, is found to consist of the siliceous 

 plates or frustules of the above-mentioned Diatomaceae, united together 



Fig. 16. 



Fig. 17. 



Fig. 18. 



Bacttlaria GalloneUa Gallondla 



•vulgaris ? distorts, ferruginea. 



These figures are magnified nearly 300 times, except the lower figure of O. ferruginea (fig. 18 a), 

 which is magnified 2000 times. 



without any visible cement. It is difficult to convey an idea of their 

 extreme minuteness ; but Ehrenberg estimates that in the Bilin tripoli 

 there are 41,000 millions of individuals of the Gaillonella distans (see 

 fig. 17) in every cubic inch, which weighs about 220 grains, or about 

 187 millions in a single grain. At every stroke, therefore, that we make 

 with this polishing powder, several millions, perhaps tens of millions, of 

 perfect fossils are crushed to atoms. 



The remains of these Diatomacese are of pure silex, and their forms 



are various, but very marked and constant in particular genera and spe- 



Fig 20 _ Fig 19- cies. Thus, in the family Ba- 



cillaria (see fig. 16), the fos- 

 sils preserved in tripoli are 

 seen to exhibit the same di- 

 visions and transverse lines 

 which characterize the living 

 species of kindred form. With 

 these, also, the siliceous spicu- 

 le or internal supports of the 

 freshwater sponge, or Spon- 

 gilla of Lamarck, are some- 

 times intermingled (see the 

 needle-shaped bodies in fig. 

 20). These flinty cases and 

 spiculse, although hard, are 

 very fragile, breaking like 

 glass, and are therefore admi- 

 rably adapted, when rubbed, 

 for wearing down into a fine 

 powder fit for polishing the 

 surface of metals. 



Besides the tripoli, formed 

 exclusively of the fossils above 



tions of a species of Gallonella, and spiculse -i •-, -, ,-, •_, .-i 



ofSpongttia. described, there occurs in the 



Fragment of semi-opal from the great bed of tripoli, Bilin. 



Fig. 19. Natural size. 



Pig. 20. The same magnified, showing circular articula- 



