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 About three inches are then to be carefully withdrawn by a pipette, when the tube may be filled up and the operation repeated.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 The gathering is thus divided into three portions, viz.--that which was withdrawn by the pipette, that which remained floating in the lower three inches of water in the tube, and that which had settled at the bottom.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. As occasional aids, it may be remarked, that in some cases _liquor ammoniæ_ may be used in place of water, as it often separates fine dirt, which is not otherwise easily got rid of. Some fossil deposits require to be treated with a boiling solution of carbonate 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.