59 
and a union of the contents takes place. A sporangial frustule 
is produced, much larger than the parent frustule ; this sporangial 
frustule divides, producing other frustules, as before described, and 
in this manner are accumulated the vast deposits previously 
mentioned. 
The fossil remains of the diatoraacefe are sometimes used in the 
arts as a polishing powder, under the name of tripoli. Another 
aTul more e.\traordinary use is made of them by savage tribes, viz., 
mixing them with flour in the time of dearth, and from this 
circumstance, the German microscopists call these deposits Berg- 
mehl and Essebar Erde. It is scarcely possible that any of the cell 
contents remain in the organisms found in these deposits, and 
they must, therefore, be perfectly useless for the purpose of 
nourishment. 
I again allude to the power possessed by vegetable organisms of 
secreting silex : this, as you are aware, is not confined to the 
simple types I have been endeavouring to describe, but is possessed 
by many of the more highly organized forms, as the horsetails, 
grasses, &c., and, apparently, the leaves of some plants are able to 
do so oven when detached from the stem. 
I have lately received from Dr. Lowe, of Lynn, a paper on 
some silicificd forms in mud from the Zambesi ] these forms are 
not peculiar to the Zambesi — they may be found in various fresh 
water deposits. Ehrenberg has figured, under the name of Phjdo- 
litharia, several of these forms, supposing them to be distinct 
organisms. Dr. Lowe has, however, been able to see them in 
situ, and they prove to be cells belonging to some tliick-leaved 
plant. Now, as no known plant possesses siliceous leaves, the 
cells must have become so after the leaves had fiillen into the 
water • and, as some of the cells retained the remains of chloro- 
phyl, it is not too much to suppose that they eliminated the silex 
somewhat as the diatom does, and must have done it rapidly. If 
these organisms had been only siliceous casts, their formation 
might have been accounted for by the cell walls acting as a dialyser, 
and separating the silica in a colloid state from the surrounding 
water, but this is not the fact — the cell walls are silicified, the 
interior remaining hollow as before. 
This power of appropriating silex from the surrounding waters 
is possessed in a firr greater degree by plants than animals, and 
