OF THE FREE DIATOMACEiE. 25 



fluid is received into several glasses. If now these are 

 examined, it will be found that they contain individuals of 

 different species, for the heaviest sorts sank first to the 

 bottom of the tube, and were the first drawn off, the 

 lighter swam longer on the surface of the water contained 

 in the tube, and were the last therefore to leave it. 



Another plan, as ingenious as it is easy to put in 

 practice, is that recommended by Eeinicke, the principle of 

 which is founded on the peculiar property, possessed by 

 the Diatomaceaa, of pressing towards the light. Having 

 collected a quantity of mud, which is seen to be overlaid 

 by a deposit of Diatomaceaa, it is spread out on a shallow 

 plate, a common dinner plate answers the purpose very 

 well. Upon this is laid a piece of thin linen, or canvass, or 

 cotton stuff, and sufficient water gently poured upon the 

 whole to cover it entirely. The plate is then left near the 

 window of a room in a clear light, or, better still, where 

 the full rays of the sun may fall upon it. The tiny 

 organisms immediately begin to creep through the meshes 

 of the overlying cloth, and, in two or three days, form on 

 it a thickish coating (free from all earthy matter), which 

 may be taken up by means of a camel's hair pencil, and 

 laid upon a slip of glass. Of course this process is avail- 

 able only so long as the plants are fresh and full of life. If 

 left too long in the room, they lose their vitality, and must 

 be got at by some other means. Another point to be 

 remembered is, that it can be employed only with certain 

 species, such as the Nitzschiae and Naviculae, whose move- 

 ments are perfectly free and active. There are numerous 

 genera in which the individuals have but slight inherent 

 powers of motion; these cannot be prepared after this 

 method. 



Gerstenberger's plan of propagating Diatomaceaa in 

 confinement, depends on nearly the same principle He 

 also spreads out the mud on a plate or shallow dish, and 

 places it near a window in the full light of the sun. 

 Stimulated by its rays, the plants begin to multiply 



