Glycerine jelly is also a valuable mountant for permanent work. When this is used the object should first be soaked in glycerine, and then in the melted jelly. It is then transferred to a drop of melted jelly which has been placed on a warm slide, and covered as before. The jelly soon solidifies, so that a ring of cement is not absolutely necessary, though it is advisable, as a rule, to cement the cover-glass all round with gold size or black varnish.
Sections cut while frozen are best mounted in glycerine, to which they may be transferred direct.
Canada balsam is one of the best media for permanent mounting; and, as it becomes very hard after a time, it serves the purposes of both preservative and cement. When this is used the object must be entirely freed from water by soaking it in absolute alcohol. It is then put into turpentine for a minute or two, transferred to a warm slide, and covered with a drop of the prepared balsam. Sections that have been imbedded in paraffin may be mounted in this way, the turpentine acting as a solvent for the paraffin in which it was cut.
Although the compound microscope is absolutely necessary for the study of the minutest forms of life and of the minute structure of the various tissues of larger beings, yet the young naturalist will find that a vast amount of good work may be done without its aid. Thus the general structure of the larger species may be made out by means of simple dissections requiring no extraordinary skill on the part of the worker, and with appliances that may be obtained at a low cost. Certain of the marine animals, however, require special treatment that can hardly be described in a short chapter devoted to general instructions only, but hints with regards to these will be given in future chapters in which the animals referred to are described.
The appliances referred to above include nothing more than a simple form of dissecting trough, a few dissecting instruments, and one or two minor accessories that may always be found at hand as required.
The dissection of animals is always best performed under water, for by this method the object examined may not only be kept clean as the work proceeds, but the parts, having a tendency to float, readily separate from one another and therefore become more distinctly visible when submerged.
Fig. 48.—Sheet of Cork on thin Sheet Lead
A very convenient form of trough may be made by taking any kind of rectangular, flat-bottomed dish, one made of zinc being, perhaps, the best of all, and covering the bottom with a slab of good cork carpet which has been weighted with sufficient lead to prevent it from floating. Or, instead of cork carpet, a sheet of cork may be used. In either case, a piece of thin sheet lead, a little larger than the slab, should be cut, the corners of which are then snipped off as shown in fig. 48, and the edges finally turned over as represented in the next illustration. The size of the trough must be regulated according to the nature of the work to be done, but one measuring ten inches long, seven wide, and two inches deep will answer most purposes.