The Microscope. Its History, Construction, and Application 15th ed. / Being a familiar introduction to the use of the instrument, and the study of microscopical science

by Jabez Hogg

VEGETABLE CRYSTALLINE SUBSTANCES.

Cuticle of Leaf of Correa Cardinalis.
" " Deutzia scabra. No. 173.
" " Elæagnus.
" " Onosma taurica.
Equisetum. No. 170.
Fibro cells from orchid. No. 169.
" Oncidium bicallosum.
Scalariform Vessels from Fern.
Scyllium Caniculum. No. 177.
Silicious Cuticles, various.
Starches, various. No. 167.

The formation of artificial crystal may be readily effected, and the process watched, under the microscope, by simply placing a drop of saturated solution of any salt upon a previously warmed slip of glass.

Interesting results will be obtained by combining two or more chemical salts in the following manner. To a nearly saturated solution of the sulphate of copper and sulphate of magnesia add a drop on the glass-slide, and dry quickly. To effect this, heat the slide so as to fuse the salts in its water of crystallisation, and there remains an amorphous film on the hot glass. Put the slide aside and allow it to cool slowly; it will gradually absorb a certain amount of moisture from the air, and begin to throw out crystals. If now placed under the microscope, numerous points will be seen to start out here and there. The starting points may be produced at pleasure by touching the film with a fine needle point, so as to admit of a slight amount of moisture being absorbed by the mass of salt. Development is at once suspended by applying gentle heat; cover the specimen with balsam and thin glass. The balsam should completely cover the edges of the thin glass circle, otherwise moisture will probably insinuate itself, and destroy the form of the crystals.

Mr. Thomas succeeded in crystallising “the salts of the magnetic metals” at very high temperatures, with very curious results. In [Plate VIII]. are seen crystals of sulphate of iron and cobalt, No. 163; and of nickel and potash, No. 165, obtained in the following manner:—Add to a concentrated solution of iron a small quantity of sugar, to prevent oxidation. Put a drop of the solution on a glass slide, and drive out the water of crystallisation as quickly as possible by the aid of a spirit lamp; then with a Bunsen’s burner bring the plate to a high temperature. Immediately a remarkable change is seen to take place in the form of the crystal, and if properly managed the “foliation” represented in the plate will be fairly exhibited. The slide must not be allowed to cool down too rapidly or the crystals will probably absorb moisture from the atmosphere, and in so doing the crystals alter their forms. Immerse them in balsam, and cover in the usual way before quite cold.

Sublimation of Alkaloids.—The late Dr. Guy, F.R.S., directed the attention of microscopists to the fact that the crystalline shape of bodies belonging to the inorganic world might be of service in medical jurisprudence. Subsequently, Dr. A. Helwig, of Mayence, investigated this subject, and found the plan applicable not only to inorganic but also to organic substances, and especially to poisonous alkaloids. By using a white porcelain saucer Dr. Guy was able to watch the process of crystallisation more minutely, and to regulate it more exactly. He was, in fact, able to obtain characteristic crusts composed of crystals of strychnine weighing not more than ¹⁄₃₀₀₀th or ¹⁄₅₀₀₀th of a grain. Morphia affords equally characteristic results. For the examination of these, Dr. Guy recommended the use of a binocular microscope with an inch object-glass. But it is not to crystalline forms alone that one need trust; the whole behaviour of a substance as it melts and is converted into vapour is eminently characteristic, and when once deposited on the microscopical slide, under the object-glass, the application of re-agents may give still more satisfactory results. The re-agents, however, which are here to be applied are not of the kind ordinarily employed. Colour-tests under the microscope are, comparatively speaking, useless; those that give rise to peculiar crystalline forms are rather to be sought after. For instance, the crystals produced by the action of carbozotic acid on morphia are by themselves almost perfectly characteristic. These experiments should not, however, be undertaken for medico-legal purposes by one unskilled in their conduct, for the effects of the reagents themselves might be mistaken by the uninitiated for the result of their action on the substance under examination. For the special method of procedure, see Dr. W. Guy, “On the Sublimation of the Alkaloids.”[34]