It would be useless to enter into particulars respecting the various salts and treatment they require, as a great difference is effected even by the strength of the solution. There are some crystals, also, which are called forth in insulated portions, showing no formation upon the ground; but even when mounted in any preserving fluid, and unchanged for a year, a new action seems to arise, and a groundwork is produced which bears little resemblance to the original crystal. Sometimes this new formation adds to the beauty of the slide; in other cases the reverse is the result, the slide being rendered almost worthless. This action, I believe, frequently arises from some liquid being contained in the balsam or other mounting medium used; and this is rendered the more probable by the crystallization being called forth in an hour after the balsam diluted with chloroform is employed, whereas no change would have taken place for months (if at all) had pure balsam been used.
Sections of some of the salts are very interesting objects, but the method of procuring these and their nature will be described in Chapter V.
The scales of various fish have been before mentioned as mounted “dry;” when, however, they are required for polarising objects they are generally mounted in balsam, and some few in liquid. The former method will be considered here.
The eel affords a beautiful object for this purpose. The scales are covered by a thin “_skin_,” which may be slightly raised with a knife and then torn off, in the same manner as the covering of the geranium and other petals, described in Chapter II. The required portion may then be removed; or if a piece of skin can be procured as stripped off in cooking, the scales may be easily taken from the inner surface. They must then be washed and thoroughly cleaned. After drying, soak for a day in turpentine, and mount in the ordinary manner with balsam. This is a good polarising object; but the interest, and I think the beauty, is increased by procuring a piece of eel’s skin with the scales _in sitû_, washing and drying under pressure, and mounting in balsam as before. The arrangement of the scales produces beautiful “waves” of colour, which are quite soothing to the eye after examining some of the very gorgeous salts, &c.