DIATOMACKffi. 



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 (strise), ribs (costae), pinnules 

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

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

 cells or areolse ; 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 

 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, a portion of the silex will be 

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



Fig. 222. 



1, Pleurosigma attmuatum. 2, Pleurosigma an 

 gulatum, magnified 250 diameters. 3 , Pleuro- 

 sigma Spencerii, magnified 350 diameters. 



