40 PREPARATION AND MOUNTING 



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 ope- 

 rated upon in the same way to separate a particular diatom. 



As occasional aids, it may be remarked, that in some cases 

 liquor ammonice may be used in place of water, as it often sepa- 

 rates fine dirt, which is not otherwise easily got rid of. Some 

 fossil deposits require to be treated with a boiling solution of car- 

 bonate of soda to disintegrate them ; but this operation requires 

 great care, lest the alkali should destroy the diatoms. Vegetable 

 silicates also sometimes require to be removed by a solution of 

 carbonate of soda ; but as the frustules of the diatoms themselves 

 are but vegetable silica, even more care is required in this case. 

 It may be well to mention, that some diatoms are so imperfectly 

 siliceous that they will not bear boiling in acid at all. Some of 

 these may be allowed to stand in cold nitric acid some time, 

 whilst others of a smaller and more delicate character should, 

 when possible, be treated with distilled water alone. 



We will now consider the mode of mounting the prepared dia- 

 toms, which, if used dry (as described in this chapter), should be 

 carefully washed two or three times with the purest distilled 

 water. In this branch, as in every other, each collector gives 

 preference to that method in which he is an adept. Thus the 

 diatoms may be placed on the under side of the cover, to be as 

 near to the object-glass as possible, or upon the slide itself; and 

 each plan has its advocates. Whichsoever of these is used, no- 

 thing seems more simple to the novice than a tolerably equal dis- 

 persion of the objects upon the slide or cover ; but this is by no 

 means so readily accomplished, consequently I give Mr. Rylands' 

 method, as his slides are perfect in this respect also. He always 

 places the diatoms upon the thin glass cover. It is not sufficient, 

 as is frequently thought, to take a drop of liquid containing the 

 cleansed material and spread it upon the cover or slide, as with- 

 out some additional precaution that uniform and regular distribu- 

 tion of the specimens is not obtained which is desirable. In order 

 to effect this, let a drop of the cleansed gathering be diluted suffi- 

 ciently for the purpose how much must be determined in each 



