[Illustration: Fig. 264.--Spring Clip for Mounting.] In mounting objects in fluid, the glass cover should come nearly, but _not quite_, to the edge of the cell, a slight margin being left for the cement, which should project slightly over the edge of the cover, in order to secure it to the cell. _Media for Preserving Algæ._--The most useful preservative media for algæ are chrome-alum, formalin, and camphor water. The solution should consist of one per cent. of chrome-alum and one per cent. of formalin; this will render the gelatinous sheath and matrix form clear, while it will retain the colour of the algæ in most cases. The Chlorophyceæ do well in any of these media; but other species, as _Ulva Lactuca_, are rendered somewhat brittle. For such use formalin alone. The Phæophyceæ should be placed while fresh in the formalin; the larger forms are better fixed by placing them for an hour or two in chrome-alum solution. The Florideæ do well in any of the three solutions, but the more delicate species, _Griffithsia_, require a two per cent. formalin solution in sea-water; the plant preserves its natural appearance in this medium. To preserve and mount diatomaceæ in as nearly as possible a natural condition, they should be first well washed in distilled water and mounted in a medium composed of one part of spirits of wine to seven parts of distilled water. The siliceous coverings of the diatoms, however, which show various beautiful forms under the higher powers of the microscope, require more care in preparation. The guano, or infusorial earth containing them, should first be washed several times in water till the water is colourless, allowing sufficient time for precipitation between each washing. The deposit must then be put into a test tube and nitro-hydrochloric acid (equal parts of nitric and hydrochloric acids) added to it, when a violent effervescence will take place. When this has subsided, the whole should be subjected to heat, brought nearly to the boiling point for six or eight hours. The acid must now be carefully poured off, and the precipitate washed in a _large_ quantity of water, allowing some three or four hours between each washing, for the subsidence of some of the lighter forms. The sediment must be examined under the microscope with an inch object-glass, and the siliceous valves of the diatoms picked out with a coarse hair or bristle.