DO 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 

 repetition and variation of this process the shaking being 

 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 fully withdrawn by a pipette, when the tube may be filled 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 inches 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 withdrawn by the pipette, that which remained floating 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 a 

 particular diatom. 



