DIATOMACEiE. 



421 



truly observes, " An organic membrane ought to exist, for 

 the silica could not become solid except by crystallizing or 

 depositing itself on some pre-existing substance." The 

 surface of the frustules is generally very beautifully 

 sculptured, and the markings assume the appearance of 

 dots (puncta), stripes (striae), ribs (costs;), pinnules 

 (pinnae), of furrows and fine lines ; longitudinal, trans- 

 verse, and radiating bands ; canals or canaliculi ; and of 

 cells or areolee ; whilst all present striking varieties and 

 modifications in their form, character, and degree of deve- 

 lopment. Again, the fine lines or striae of many frustules 

 are resolvable into rows of minute dots, such as occur in 

 Pleurosigma, &c. 



The nature of the markings on the Diatom valves is one 

 of considerable in- 

 terest, and has ex- 

 cited much atten- 

 tion, and attempts 

 have been made to 

 produce them arti- 

 ficially. On the ad- 

 dition of sulphuric 



acid to a mixture of ffl H B HI 



powdered fluor spar 

 and sand, an imme- 

 diate evolution of 

 fluoride of silicon 

 takes place, as is 

 shown by the white 

 fumes. This white- 

 ness is due to the 

 presence of minute 

 particles of silex 

 arising out of the 

 decomposition of the 

 fluoride by the mois- 

 ture contained in the 

 atmosphere ; and il 

 a solid body be ex- 

 posed to these vapours 





Fig. 222. 



Pleurosigma attenuatum. 2, Pleurosigma an 

 gulatum, magnified 250 diameters. 3, Pleuro- 

 sigma Spencerii, magnified 350 diameters. 



a portion of the silex will be 



deposited on it, in the form of a fine white powder, con- 



