Page:Hardwicke's Science-Gossip - Volume 1.pdf/132

Proposal to London Microscopists.—It appears to me that some association amongst the amateur microscopists of London is desirable, which shall afford greater facilities for the communication of ideas and the resolution of difficulties than the present Society affords, and which, whilst in no respect hostile to the latter, shall give amateurs the opportunity of assisting each other as members of an amateur-society, with less pretensions, holding monthly meetings in some central locality, at an annual charge sufficient to cover the incidental expenses—say five shillings a year—on the plan of the Society of Amateur Botanists. By the publication of this letter the general feeling of the parties interested will be ascertained, and by this future action determined.—W. Gibson.

N.B.—We insert our correspondent's communication, and would be glad to hear from any microscopists desirous of co-operating with him in carrying out his proposition.—Ed. Sc. G.

Mounting Opaque or Transparent Objects.—In reference to a mode of mounting for opaque objects I have found that, after trying cardboard, gutta percha, india-rubber, ivory, moulded shellac, &c., none answer my purpose so well, or look so neat, as those which are to be made from fishing-rod rings. My modus operandi is as follows: With a common gun-wad punch, punch out the paper, to form the back-ground, a little less than the ring, but slightly larger than its internal diameter. The paper need not be dead black, as, if so, its texture will be so coarse as to show its roughness with a 2-inch objective. A very dark bronzed highly-finished paper will be found the best. Have ready a number of the rings, blacked inside with Brunswick black, those measuring ½-inch external diameter being most useful. Next, centre a slip of glass on your turn-table, and with a little gum fix the paper in the centre of the slide, and by the time that you have thus papered, say six dozen, the first slip will be ready for the next stage, when you again centre on the turn-table, and just on the outside, but touching the paper, run a thin ring of gold-size, upon which drop the prepared cell; and so on with the remainder. When all are done, place them on wooden trays, and bake them on the top of the oven. In the course of a day or two again gold-size the cell, letting the size extend only a short distance on the glass beyond the circumference of the cell. When this second coat is dry, a third of Brunswick black may be laid over the whole exterior, filling up, as much as possible, the angle formed by the outer wall of the ring and the surface of the glass. In placing the object, a sufficiently small portion of gum may be generally taken up with a fine brush, just enough to make the object adhere firmly, and yet not to show beyond its margin. In fixing the glass cover again use gold-size, as being probably the most tenacious cement there is,a nd as being less liable to "run in" than the majority of such. After one or two applications, made at longish intervals, the whole may be finished off with Brunswick black. I say the applications are to be made at longish intervals, because I find that most of my friends are in far too great a hurry with their work, puting on coat after coat long before the last application can possibly be dry. Brunswick black and other brittle cements should never be used where adhesion is of the first consideration. As to the mounting of transparent objects, I am now using the glycerine jelly, recommended by Mr. Lawrence in the Microscopical Journal, and referred to in that, to the working microscopist, indispensable little brochure, "The Preparation and Mounting of Microscopic Objects," by Mr. Davies, and in which work the reader will find instructions for its employment. I can certainly recommend it most highly as being easily workable, clean, a most excellent preservative, and the object quickly put up and finished; the latter being a great desideratum with novices. If it had no other advantage there is one greatly in its favour, viz., that there is no fear of leakage. Whilst confervoid growths—to which, it is said, the jelly is somewhat liable, though I have not found it so—might probably be entirely avoided by always keeping the bottle closely corked, and carefully sealing the object mounted by a ring of cement.—J. E. W.

Granite.—Granite, generally and theoretically, consists of crystals of quartz, of felspar, and of mica embedded in a crystalline quartz. Practically, all porphyritic rocks are called granites. In many of these rocks the mica is replaced by hornblende, the result being the variety called Syenite. The mica and hornblende are sometimes absent or present in very small proportions. The quartz sometimes preponderates, and is sometimes almost absent. The felspar is sometimes in distinct crystals and sometimes only crystalline. The felspathic portion of the stone also is sometimes felspar proper, but occasionally albite, in which the alkaline element is chiefly soda instead of potash. The crystals vary in size and in the mode of their distribution, so that the granite may be coarse or fine grained. Some kinds of granite are brittle, and others singularly tough; some very easily break along lines of natural fracture, and are comparatively soft while yet fresh from the quarry, while other kinds resist any regular fracture, and are only reduced to a desired shape with extreme difficulty, even when first obtained. Chemically, true granite is a silicate of alumina and potash, with a little iron and lime, and with soda sometimes replacing the potash and magnesia the