The gathering may then be transferred to a small boiling glass or test-tube, and--the water being carefully decanted--boiled in the smallest available quantity of nitric acid, and washed as before. This last process has been found necessary from the frequent appearance of minute crystals, which cannot otherwise be readily disposed of without the loss of a considerable proportion of diatoms. [B] These glasses are round, about six inches high, and usually contain about eight ounces. They are rather wider at the bottom, tapering gradually to the top, and may be generally procured at the chemists, &c. I may here mention that the washing glasses used by Mr. Rylands are stoppered conical bottles varying in capacity from two ounces to one quart; the conical form being employed to prevent the adherence of anything to the side; they are “stoppered” to render them available in the shaking process about to be described. The gathering, freed from acid, is now put into two inches depth of water, shaken vigorously for a minute or two, and allowed to subside for half an hour, after which the turbid 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 examined 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 quantity 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 contains a variety of species and forms, it may require to be divided into _different densities_.