OF THE FREE DIATOMACEJS. 
21 
separated themselves, and sunk to the bottom. This will 
take place in about half a minute. The fluid is next 
carefully decanted into a second glass, and the amount of 
water lost made up, the same process being gone through 
five or six times. The sediment in the second glass now 
contains all the Diatomacea?, and sandy particles, which 
were too light to subside during the first half minute. To 
separate these still further, this sediment is treated exactly 
as was the original gathering, excepting that a longer time 
is given it (say two minutes and a half), between each 
decanting. In the same way the contents of the third 
glass are sorted, only that a still longer period, not less 
than five minutes, is allowed. Every glass now contains 
samples of Diatomacege, mingled with earthy ingredients of 
varying weights, the last glass having of course the 
lightest. To remove the foreign bodies altogether, each 
mass of sediment is placed in turn in a short wide glass, 
and about an inch height of water is poured over it, where 
it is left for a minute or two. As soon as the whole of the 
contents may be supposed to have settled, a rotatory 
motion is given to the water, by moving the vessel with 
the hand in a circular direction. The Diatomacese, con- 
sisting mostly of thin plates or valves, are raised by the 
agitation of the water to the surface, while, on the other 
hand, the sandy particles being more or less round, are 
rolled upon each other at the bottom, and gradually collect 
in the centre of the vessel. The fluid is now quickly, but 
with great care, decanted into a second glass, where the 
Diatomacese are allowed to subside. What remains in the 
first glass may be again and again submitted to the same 
process, until it is tolerably certain that not a single 
Diatom has been overlooked. 
In order to effect a separation of the different species, 
by taking . advantage of their varying specific gravities, 
the following method, introduced by Munro, is excellent, 
and has the merit of great simplicity. A glass tube, 
three to four feet in length and half an inch wide, is 
