284 



THE POPULAR SCIENCE MONTHLY. 



charcoal or soot ; and reasons upon the sub- 

 ject as follows : The hand-rails on the stair- 

 cases of the Metropolitan Railway stations, 

 after sonic use, become coated with a deli- 

 cate 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 ques- 

 tions is, that it consists of a selection of the 

 very finest particles of London soot. The 

 hands of passengers in rubbing along the?e 

 rails conduct a debtor and creditor trans- 

 action. There is sootcarbon on the hands 

 and soot-carbon on the rails, as on every- 

 thing, 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 par- 

 ticles 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 lamp- 

 black or ordinary soot, the finest vegetable- 

 black, and powdered plumbago, arc rubbed 

 upon paper, the appearance of the fine black 

 will be found to be intermediate between 

 that of the other two substances. 



What Mic Edible Birds' Nests are made 



oft — Naturalists have not been able to de- 

 cide of what material the edible birds' nests 

 are composed. Some have regarded them 

 as made of pure animal secretions ; others 

 believe that algai enter largely into their 

 composition. Mr. E. L. Layard has sug- 

 gested that the nests of the first quality, or 

 those which are made early in the season, 

 are made of secretion, but that later on, if 

 the first nests arc 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 vari- 

 ous qualities, and finds them all to become 



alike gelatinous in texture on soaking, and 

 made up of laminaa affixed by their faces to 

 one another. Some nests of inferior qual- 

 ity showed the presence of algae, but neither 

 in their mode of disposition nor in the quan- 

 tity did they confirm Mr. Layard's view. 

 The scanty amount and rrcgular position 

 of the algie 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 

 algaj 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 va- 

 rious 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 Strombiis, natives of the West In- 

 dies, 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 ren- 

 der 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 pen- 

 cil upon it ; and from him it passes to an- 

 other 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 



