ALG.^. 185 



which hVing organisms may develop themselves in 

 the atmosphere." He supposes it probable that the 

 atmospheric dust-cloud region is of vast extent, and 

 is above a height of 14,000 fect.^ 



Whilst accepting the facts determined by Ehren- 

 berg, we are far from adopting his theory of a vast 

 dust cloud in " which living organisms may be de- 

 veloped." This is undoubtedly a romance which has 

 no substantial basis in reason or in fact. Against 

 the assumption that these minute organisms have 

 been swept up by the force of wind, or currents of air, 

 held for some time suspended, and finally been pre- 

 cipitated in clouds, like ' showers, no very strong 

 argument can be adduced. The evidence is too 

 strong to be rejected, that showers have been from 

 time to time precipitated, and that, in cases where 

 samples of that dust could be obtained, and submitted 

 to examination, it has been found to consist, in very 

 large proportion, of organized matter, and much of 

 that consisting of the frustules of minute Diatomaceae. 



Fossil Diatoms. 



None of the lower forms of vegetable life are more 

 common or more widely diffused than the Diato- 

 maceae. These were at one time, on account of their 

 spontaneous movements, called Bacillaria and Infu- 

 soria, and suspected to belong to the animal kingdom. 

 More complete investigation, however, determined 

 them to be algse, having a silicious or flinty skeleton. 

 Typically, two similar thin translucent plates of silex, 



^ " On the Infusoria, and other Microscopic Forms in Dust 

 Showers," by C. G. Ehrenberg, in Siliiman''s /ournal (1851), xi. p. 372. 



