126 



NA TURE 



[_Dc 



1884 



scale, would answer for a searcher, and the motions could be 

 given by simple mechanical means or by means of electromotors 

 worked from any point. Here the chief working parts of the 

 apparatus would be fully protected, and this would be of the 

 first importance, and the rapidity with which the light could be 

 directed to any point or rendered quite invisible would be a 

 great improvement on the present model, where all has to be 

 exposed. 



For forts requiring powerful searchers, and it is easy to see 

 thai they might be of great use here, this arrangement is suit- 

 able, particularly as the mark, being stationary, is more likely to 

 be struck than in a ship ; but the replacing of the plane mirror 

 would be easily effected, and other part of the apparatus of 

 course being quite protected, as in the case of ships. 



In the case of a fort in a channel that it was desired to pro- 

 tect, the beam of light from a powerful fixed parabolic reflector 

 ould be so truly sent that it could be reflected from mirrors at 

 a distance, as on the banks of the channel, so as to sweep across 

 close above the level of the water and show the smallest object 

 crossing the illuminated line. 



It may be objected that in this second reflection there will be 

 a loss of light, but that loss can be made very small, and there 

 would be positive gain 111 using a large parabolic mirror in place 

 of the necessarily small and imperfect ones in use in a lantern 

 of a lighthouse or the deck of a vessel. Such a parabolic 

 ild be made accurately in sections of very thin glass 

 silvered at the back so as to retain its reflectin ivei I 11 

 indefinite time ; in the case of a lighthouse it might be placed at 

 any point vertically below the lantern, even at the bottom if the 

 tower had a well as large as the intended beam of light. The 

 large mirror above may be also of thin glass silvered in a similar 

 way and with such a slight curvature as might be required to 

 enlarge I min any way, and more than one of these mirrors 



might be used if it was necessary to have a fixed light in one 

 constant direction or for any other purpose. I am not sure if 

 there would be any gain in the power to penetrate fog. In the 

 caseof a head-light, there would lie certainly, from tb< 

 of light into a beam instead of the naked arc; but whethei a 

 as the very small point thai forms the arc including 

 the incandescent carbon ceases to affect the eve in : 

 than the same intrinsii a surface must only be 



settled by experiment on a proper scale. 



Ealing, Decembei 5 A. Ainslie Co 



Natural Science in Schools 



In the interesting discussion which has recently been carried 

 on in your pages on the teaching of natural science i 

 not much has he 11 said about the text-books which are, or 

 ■.hould be, read. So long as the present system of teaching a 

 single branch of natural science continues, and until the method 

 recommended by Prof.. Armstrong is adopted, it is deal thai 

 . ne sh iuld he exercised in the choice of a good text-book 

 on the particular subject selected. Even when it is found pos- 

 sible to teach science in the form of physiography, m .'. 



kunde, there will doubtless be many boys in the large scl Is 



who, having thus obtained a great amount of mosl valuabli 



al knowledge and a wider view of the aims of 

 than is possible under the present system, will wish to carry 

 on their studies in a particular direction. Taking chemistry, 

 as the subject with which I am most familiar, and which at 

 present is perhaps more widely taught than am other branch of 

 I ■•;, be tid th . n should be no difficulty at all in 

 selecting a suitable hook. It is true that the number of text- 

 book of chemistry is extremely large, and it is also true that 

 there are a few hooks, written by men of wide knowledge and 

 long experience in teaching, which are well adapted to the 

 purpose in view. But it is, unfortunately, equally true thai 

 there are many text-books which are either untrustworthy 01 are 

 badly arranged, or which contain little more than a bare collec 

 tion of dry facts, and it is to be feared that some of thi 

 unfrequently find their way into schools. Doubtless most 

 teachers of chemistry will agree with Prof. Armstrong that the 

 educational value of a course of instruction dealing men 

 the methods of preparation and the pro] 1 rties of a number of 

 elements and compounds is extremely small, because the faculty 

 of reasoning from observation is not thereby develop d. It will 

 il o, ] think, be generally admitted that " it is of great import- 

 ance that the meaning of the terms •equivalent,' 'atomic 

 weight,' 'molecular weight,' should be thoroughly grasped al 



an early stage." But it would perhaps be better that students 

 should remain in complete ignorance of the meaning of these 

 terms than that they should obtain such erroneous and illogical 

 notions of atoms and molecules as are contained in some of the 

 text-books. One of these books, which in 1S80 had passed 

 through no less than fifteen editions, and which appears there- 

 fore to be largely read, anil which is advertised as being recom- 

 mended by the head-masters of certain schools, contains the 

 following remarkable statements : — 



" Chemists assume that the elementary bodies are built up of 

 infinitely small particles, which they call atoms ; they further 

 assume that these atoms, with few exceptions, are all of the 

 same size. . . The exceptions are phosphorus and arsenic, whose 

 atoms are believed to be half the usual size ; and zinc, cad- 

 mium, and mercury, whose atoms are double the size." (The 

 italics are the author's.) To the uninitiated it might appear 

 strange to argue about the relative sizes of infinitely small 

 particles. 



Again : — " All molecules arc of the same she ; for the law of 

 Ampere, which most chemists now accept, states that ' all gases 

 and vapours contain the same number of molecules within the 

 same volume.'" 



Most of the errors contained in these statements are of course 

 due to a misapprehension of the meaning of Avogadro's (Am- 

 pere's) law. It is not very easy to give an average student a 

 clear conception of the fundamental generalisations and theories 

 by means of which chemists have been able to determine the 

 most probable relative atomic weights of the elements. To do 

 this, it is first of all necessary to induce the student to think and 

 tea -in for himself, and it seems to be much easier for most 

 people to repeat a thing from memory than to understand it. 

 But when the student's memory has already been stocked with 

 such illogical statements as those quoted above, the difficulty is 

 very greatly enhanced. Sydney Young 



University College, Bristol 



The Edible Birds-Nest 



The nature of the material from which the edible bird's-nest 

 is formed has been long the subject of controversy. It is very 

 gratifying to find from air. Layard's letter, published in last 

 week's Nature (p. S2), that a reconciliation of the various views 

 is possible. Most writers support the theory that the substance 

 is secreted in some way by the bird, though they differ as to the 

 manner. Sir E; Home, in a paper published in the Phil. 

 Trans., 1S17, suggests certain gastric glands as the active ones. 

 Bernstein, forty years later, points tothe prominence in the nest- 

 building season of certain salivary glands which form cushions 

 by the side- of the bird's tongue, and suggests that these secrete 

 the material. On the other hand, there are advocates of the 

 view that the nest is constructed of certain vegetable matter 

 found by the birds in the caves where the nests are built, and 

 agglutinated by them by a buccal or salivary secretion. 



Through the kindness of Prof Michael Foster I have been 

 enabled to make some observations on the chemical nature of 

 the material of the nests used for soup at the recent Health 

 Exhibition, and from rny experiments I have come to the con- 

 clusion that this is a substance resembling very closely the mucin 

 described by Eichwald, Obolensky, and other writers, as forming 

 the chief constituent of the mucous secretion of all animals and 

 les of Helix pcmati.i, &c. It shows under the micro- 

 scope scarcely any structure, but is laminated, shells splitting oft 

 easily in two directions. It contains here and there certain 

 bodies resembling the cells of squamous epithelium. It is in- 

 soluble in either cold or warm water, but swells up in either, 

 forming a gelatinous-looking mass ; in both lime-water and 

 baryta-water it is slowly dissolved, and the reactions of the solu- 

 tion differ very little from those described by the writers named 

 as those of mucin. It resembles this body also in its behaviour 

 when heated with acids, alkalies, and the different digestive 

 ferments. The solution in lime-water contained a little debris. 

 which proved to consist largely of pieces of feathers, with a little 

 adherent amorphous matter. With the exception of certain 

 microscopic particles among this, I could not get any evidence 

 of the presence of vegetable matter in the nest substance. 

 Indeed all the experiments I have described point certainly to 

 the absence of anything but a glandular secretion. 



Jos. R. Green 

 Physiological Laboratory, Cambridge, December I 



