* 66 PREPARATION AND MOUNTING 
fluid must be carefully decanted. This operation must be 
repeated until all the matter is removed which will not 
settle in half an hour. The fluid removed should be ex- 
amined by a drop being put upon a slide, as in some cases 
very light diatoms have been found to come off almost 
pure in one or more of these earlier washings. The quan- 
tity of water and time of subsidence given may be taken 
generally, but may require to be modified according to 
circumstances and the judgment of the operator. By the © 
the most important part—the gathering, if a pure one, will 
be sufficiently clean. If, however, it contain a variety of 
species and forms, it may require to be divided into different 
densities. 
In some cases, however, it is best to divide the gathering 
as a preliminary operation, which may be done by agitating 
it in a quantity of water and decanting what does not 
readily subside. The heavier and the lighter portions are 
then to be treated as two separate boilings. But when the 
cleansing has been carried to the above stage and this 
division is required, the plan must be somewhat as follows :— 
The gathering must be shaken in a test-tube with six inches 
of water, and then allowed to subside until one inch at the 
top remains pure. About three inches are then to be care- 
fully withdrawn by a pipette, when the tube may be filled 
up and the operation repeated. The three lower inchés also 
may then be decanted and examined. The gathering is 
thus divided into three portions; viz—that which was 
withdrawn by the pipette, that which remained floating © 
in the lower three inches of water in the tube, and 
that which had settled at the bottom. An examination of 
these will inform the operator how to obtain that particular 
density of gathering which he desires, and how far it is 
worth while to refine this process of elutriation; for in 
cases of necessity any one, or all three, of these densities 
may be operated upon in the same way to separate & 
particular diatom, 
