PRESERVATIVE LIQUIDS, ETC., PARTICULARLY WHERE CELLS ARE USED. There are many objects which would lose all their distinctive peculiarities if allowed to become dry, especially those belonging to the fresh-water Algæ, many animal tissues, and most of the very delicate animal and vegetable substances in which structure is to be shown. These must be preserved by immersion in some fluid; but it is evident that the fluid must be suited to the kind of matter which it is intended to preserve. As it often requires much study and trouble to _obtain_ microscopic objects of this class, it is well that their _preservation_ should be rendered as perfect as possible; and for this reason the CELLS, or receptacles of the fluids, should be so closed that all possibility of escape should be prevented. The accomplishment of this is not so easy a matter as it might appear to the inexperienced. Before giving any directions as to the manipulation required in mounting the objects, we must consider the different _liquids_ and _cells_ which are requisite for their preservation. Of the former there are a great number, of which the principal may be mentioned. DISTILLED WATER is strongly recommended by many for Diatomaceæ and other Protophytes. It has been, however, stated that confervoid growths often disturb the clearness of the liquid, and on this account various additions are made to it. A lump of camphor is often left in the bottle, so that the water may dissolve as much as possible. One grain of bay-salt and one of alum are added to each ounce of water; or a drop or two of creosote shaken up with the ounce of water, which should be afterwards filtered. These additions are often made; perhaps each of them good for certain objects. GLYCERINE.--Some affirm this to be one of the best preservative liquids, especially for vegetable objects; but others think that it is much better when diluted with two parts of camphor-water, prepared as above.[E] [E] Dr. Carpenter says:--“Glycerine has a solvent power for carbonate of lime, and should not be employed when the object contains any calcareous structure. In ignorance of this fact, the author (Dr. C.) employed glycerine to preserve a number of remarkably fine specimens of the pentacrinoid larva of the Comatula, whose colours he was anxious to retain; and was extremely vexed to find, when about to mount them, that their calcareous skeletons had so entirely disappeared, that the specimens were completely ruined.” GLYCERINE AND GUM.--This is also believed to be a very good liquid for vegetable tissues, and is thus prepared:-- Pure gum-arabic 1 oz. Glycerine 1 ” Water (distilled) 1 ” Arsenious acid 1½ grains. Dissolve the arsenious acid in the cold water, then the gum, add the glycerine, and mix without bubbles.