Page:Popular Science Monthly Volume 32.djvu/296

284 charcoal or soot; and reasons upon the subject as follows: The hand-rails on the staircases of the Metropolitan Railway stations, after some use, become coated with a delicate film of graphite or "black-leaded." The same is seen, but more faintly, on suburban hand-rails, but not at all in rural stations. "Whence comes this graphite film? Why is it developed as we approach the center of London, reaching its maximum in the most densely populated and sootiest regions of the metropolis? My answer to these questions is, that it consists of a selection of the very finest particles of London soot. The hands of passengers in rubbing along these rails conduct a debtor and creditor transaction. There is soot carbon on the hands and soot-carbon on the rails, as on everything, animate or inanimate, that is exposed to a London atmosphere. Some of the soot-particles on the rails are brushed off by the hands, some rubbed down and smeared on the wood; some are abstracted by the hands, and some are contributed by the hands as additions to the smearing. It is obvious that in such proceedings the coarser particles are those that will be brushed off or carried away, while only the very finest, the impalpably minute particles, will adhere as a black, varnish-like, unctuous film to the hard wood." So, when the coarsest lampblack or ordinary soot, the finest vegetable-black, and powdered plumbago, are rubbed upon paper, the appearance of the fine black will be found to be intermediate between that of the other two substances.

What the Edible Birds' Nests are made of.—Naturalists have not been able to decide of what material the edible birds' nests are composed. Some have regarded them as made of pure animal secretions; others believe that algae enter largely into their composition. Mr. E. L. Layard has suggested that the nests of the first quality, or those which arc made early in the season, are made of secretion, but that later on, if the first nests are destroyed, the birds can not replace them by this secretion alone, and have to use extraneous substances to help in the construction. Mr. J. R. Green, of the Physiological Laboratory, Cambridge, has examined specimens of the nests of various qualities, and finds them all to become alike gelatinous in texture on soaking, and made up of laminæ affixed by their faces to one another. Some nests of inferior quality showed the presence of algæ, but neither in their mode of disposition nor in the quantity did they confirm Mr. Layard's view. The scanty amount and rregularirregular [sic] position of the algæ would be better accounted for on the theory of their being accidental constituents. The nest-substance gave no micro-chemical reactions that could connect it at all with cellulose, so that it could not be formed by the partial digestion of the algæ and regurgitation of the resulting matter. On the other hand, it did give very striking evidence of its close relationship with the substance mucine described by various authors, and well known as a product of the animal body.

Cameos.—Cameos are made from pieces of sea-shell, of which, as every one must have noticed, while the outside is often rough and unseemly, the interior is perfectly polished, and sometimes of a brilliant color. The shells, which are usually of a species of Cassis and Strombus, natives of the West Indies, are chosen on account of the thickness and hardness of the layers, of the contrast of color between them, and of the presence of knobs on the exterior surface which render it possible to work in relief. When a cameo is begun, a piece of the shell, rather larger than the ornament is intended to be, is cut out and affixed to a holder by a kind of coarse sealing-wax. The inner surface of the shell is of course the lowest, and on the gray outside the master draws a rough outline of the design, and places the work in the hands of an apprentice, who, by means of a file, reduces the knob to the requisite height, removes all the gray matter that lies outside of the boundary-lines, and dresses the whole of the irregular surface. In this condition a cameo looks like an irregular piece of chalk rising out of a small plate of colored glass. It is now returned to the master, who again draws the design in pencil upon it; and from him it passes to another apprentice or workman, who brings out the design with a burin. In late years it has become the fashion to have cameo portraits taken, but the likenesses, to which the artist usually manages to give a classical