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 Reinicke, the principle of 
which is founded on the peculiar property, possessed by 
the Diatomacea?, of pressing towards the light. Having 
collected a quantity of mud, which is seen to be overlaid 
by a deposit of Diatomacete, 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 Nitzschias and Navicular, 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 Diatomacese 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 
