The Infusoria (see Chapter III.) are sometimes preserved in liquid; but present many difficulties to the student. Different kinds require different treatment, and consequently it is well, when practicable, to mount similar objects in two or more liquids. Some are best preserved in a strong solution of chloride of calcium, others in Thwaites’ liquid, whilst a few keep their colour most perfectly when in glycerine alone. Many of them, however, are so very transparent that they present but faint objects for ordinary observance. The Desmidiaceæ require somewhat similar treatment, and may be mentioned here. The solution of chloride of calcium has been strongly recommended; but no preservative liquid seems to be without some action upon them. Both of the above classes of objects should be mounted in shallow cells, so as to allow as high a microscopic power as possible to be used with them. Many of the ZOOPHYTES which are obtained on our sea-coasts are well preserved by mounting in cells, in the manner before mentioned, with Goadby’s fluid, or distilled water with one of the additions noticed amongst the preservative liquids. For examination by polarized light, however, they are usually mounted in balsam (see Chapter III.), whilst those in cells present a more natural appearance as to position, &c., for common study. As to the use of preservative liquids with the Diatomaceæ there are various opinions. Some experienced microscopists say that there is little or no satisfaction in mounting them in this way. Dr. Carpenter, however, explains this difference by his instructions as to what method should be used when certain ends are desired. He says: “If they can be obtained quite fresh, and it be desired that they should exhibit as closely as possible the appearance presented by the living plants, they should be put up in distilled water within cement-cells; but if they are not thus mounted within a short time after they have been gathered, about a sixth part of alcohol should be added to the water. If it be desired to exhibit the stipitate forms in their natural parasitism upon other aquatic plants, the entire mass may be mounted in Deane’s gelatine in a deeper cell; and such a preparation is a very beautiful object for the black-ground illumination. If, on the other hand, the minute structure of the siliceous envelopes is the feature to be brought into view, the fresh diatoms must be boiled in nitric or hydrochloric acid” (which process is fully described in Chapter II.). It is very convenient to have many of these objects mounted by two or more of the above methods; and if they are to be studied, this is indispensable. Mr. Hepworth once showed me about one hundred slides which he had mounted in various ways, for no other purpose than the study of the fly’s foot.