Amongst the various classes of microscopic objects now receiving general attention, the Diatomaceæ may be placed in a prominent position. They afford endless opportunities of research, and some very elaborate works have already been issued concerning them. Professor Smith’s may be mentioned as one containing, perhaps, the best illustrations. The young student may wish to know what a diatom is. The “Micrographic Dictionary” gives the following definition:--“A family of confervoid Algæ, of very peculiar character, consisting of microscopic brittle organisms.” They are now looked upon by almost all of our scientific men as belonging to the _vegetable_ kingdom, though some few still assign them to the animal. They are almost invariably exceedingly small, so that the unaided eye can perceive nothing on a prepared slide of these organisms but minute dust. Each separate portion, which is usually seen when mounted, is termed a “frustule,” or “testule:” this consists of two similar parts, composed of silica, between and sometimes around which is a mass of viscid matter called the “endochrome.” They are found in almost every description of water, according to the variety: some prefer sea-water, others fresh, and many are seen nowhere but in that which is a mixture of both, as the mouths of rivers, &c. Ditches, ponds, cisterns, and indeed almost every _reservoir_, yield abundance of these forms. They are not, however, confined to “present” life; but, owing to the almost indestructible nature of their siliceous covering, they are found in a fossil state in certain earths in great abundance, and are often termed “fossil Infusoria.” Upon these frustules are generally to be seen lines, or “markings,” of different degrees of minuteness, the delicacy of which often serves the purpose of testing the defining power of the object-glasses. Some of the frustules are triangular, others circular, and, indeed, of almost every conceivable shape, many of them presenting us with exquisitely beautiful designs. The markings, however, are seldom seen well, if at all, until the frustules are properly prepared, the different methods of accomplishing which will be given a little further on.