Many diatoms are seen best when mounted in a dry state, the minute markings becoming much more indistinct if immersed in liquid or balsam; and for this reason those which are used as test objects are usually mounted _dry_. Many kinds also are now prepared in this way, as opaque objects to be examined with the lieberkuhn, and are exquisitely beautiful. Others, however, are almost invariably mounted in balsam; but as these will be again referred to in Chapter III., and require the same treatment to fit them for the slide, it will not be out of place to describe the cleaning and preparation of them here. As before stated, there is much matter surrounding them which must be got rid of before the “_siliceous_ covering” can be shown perfectly. As, however, we may first wish to become acquainted in some degree with what we have to do, it is well to take a small piece of _talc_, and place a few of the diatoms upon it. This may be held over the flame of the spirit-lamp until all the surrounding matter is burnt away, and a tolerable idea may be thus obtained as to the quality of our treasure. In some cases it is well to use this burning operation alone in _mounting_ specimens of diatoms, when they may be placed in their natural state upon the thin glass, burnt for awhile upon the platinum plate, hereafter described, and mounted dry or in balsam. In the preparation and cleaning of Diatomaceæ, there is little satisfaction unless these operations have been successfully performed, as a very small portion of foreign matter seriously interferes with the object. The mode of preparing them varies even amongst the most experienced. It will be found, therefore, most satisfactory to examine the principal of these separately, although it may be at the risk of some little repetition.