June, iqo6.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



4G1 



mented. Professor Bragg last year showed that the " stopping- 

 power " of an atom of any gas for the Alpha streams was a 

 constant of the atom, unaffected by its association with other 

 atoms in molecular strncturc, and independent of temperature 

 and pressure. The second of these statements implies that if 

 we know the stopping-power of the constituents of a compound 

 gas we find the value for the gas itself by simple addition. 

 Not only so, but the stopping-powers are very nearly propor- 

 tional to the square roots of their atomic masses, so that a 

 simple, if approximate, law covers all the phenomena. Pro- 

 fessor Bragg has since extended these observations ; and 

 shows, moreover, that not only are the stopping-powers 

 systematic, but show also a systematic deviation from the 

 square-root law. For whilst depending mainly on the square 

 roots of the atomic masses, they have a leaning towards the 

 masses themselves. 



This is shown by the following numbers selected from a 

 much larger table in the Philosifhicdl Mai^uzine for May. 



Propor tional to 

 v'Atomic Mass. 



Hydrogen . . 

 Carbon dioxide 

 CH,I . . . . 



•243 

 1-47 

 2-58 



■264 

 1-51 

 2-35 



•242 

 I 48 

 2-52 



In each of these columns the value for air must be taken as 

 unity. The table shows clearly that closer correspondence 

 with experiment is brought about by introducing the additional 

 term. 



The same law holds for substances in the solid state ; thus 

 the stopping-power is independent of the mode of aggregation. 

 Now the Alpha particle expends energy in causing the expulsion 

 of electrons from the atoms of any gas which it Inaverses. Since 

 the expenditure follows the same law when the particles are 

 massed together into a solid, it would seem that the solid must 

 also become ionised, just as the gas is, and we should expect 

 slow-moving electrons to be projected from Radium itself and 

 from both sides of any solid screen through which the particles 

 pass. These may be the slow electrons discovered by J. J. 

 Thomson. This at least is Bragg's conclusion, and it is in 

 conformity with Soddy's opinion. It must, however, be also 

 borne constantly in mind that the slow-moving electrons supply 

 the deficit which would otherwise appear of negative elec- 

 tricity. The emanation throws off a positive charge, and yet 

 becomes itself positively charged (for it moves toward the 

 lu'i^ati'.'c terminal). Where is the corresponding negative 

 charge if not m the more recently discovered slowly-moving 

 electrons ? 



Standard Wavelengths of Light. 



It is now generally recognised that lh(^ cfliciency of a 

 diffraction-grating in the measurement of wave-lengths has 

 been very much over-rated. Different first-class gratings give 

 different values. A warning thrown out by Lord Rayleigh as 

 early as 1S8S against exaggerated expectations; there is now 

 no doubt that tlie warning was justifiable. Attention has been 

 directed in the last few years to methods based on the inter- 

 ference between the different beams which emerge after mul- 

 tiple reflections through a layer of air bounded by the two 

 plain parallel surfaces of slabs of half-silvered glass. Methods 

 such .as this are usually distinguished as iiitcrfcnncc methods 

 (such an arrangement is called an intcrfcvomftcr) ; but Lord 

 Rayleigh is very emphatic in saying that such methods have 

 no more claim to the title than the diffraction grating itself. 

 In each method final effect is obtained by focussing to a point 

 on a .screen (or focal plan of an eye-piece) a succession of 

 beams whose phases are different owing to the different paths 

 travelled. The essential feature which distinguishes the 

 parallel plate method is the greater simplicity which allows 

 greater .accuracy in construction. 



Lord Rayleigh has modified the original arrangement of 

 Fabry and Perot in the direction of simplification without 

 apparently any loss of accuracy ; and he has measured several 

 of the leading standard reference lines in various spectra. It 

 is noteworthy that in all cases Fabry and Perot's values are 

 verified to irilliiti one part in a million and in some cases to 

 considerably less. This is a most important result, since 

 allhougli it was well recognised that their values had been 



made with the greatest care and precision, yet they stood on 

 their own merits. The present confirmation, by a modified 

 method, proves that they deserved all the reliance which 

 physicists were disposed to rest upon them. 



Directed Wireless Telegraphy, 



In tlie ordinary systems of wireless telegraphy there is no 

 means of knowing the direction from which the signals have 

 arrived, or to send signal in one direction only. Theoreti- 

 cally it would be possible by means of a sufficiently large 

 i parabolic mirror to concentrate all the radiation along a single 

 path ; hut the size of the mirror necessary for this would 

 be heroic. This arises froin the fact that very long waves arc 

 employed ; for such waves the mirror must be proportionately 

 large. It has now been found out by Signor Marconi and 

 those working with him that when the " aerial " is placed hori- 

 zontally instead of vertically, and with the spark at the end 

 away from the sending station, a similar receiver picks 

 up far more energy when the two are in one straight line. In 

 this arrangement it is clear that it must be the magnetic com- 

 ponent of the wave which is most efficient in producing the 

 signal. This discoverv would seem to mark a distinct advance 

 in this important pr.ictical subject. 



ZOOLOGICAL. 



I;'>y R. Lydekker. 

 Habits of the Dugong. 



Some interesting notes on the habits of the Indian 

 iluf;ong, or sea-cow, are published by N. Annand.-ile, of the 

 Indian Museum, Calcutta, in the Journal of the Asiatic 

 Socirty nf liencjal, for last year. After stating that dugong 

 feed largely on a green alga, as well as on a marine 

 |ibanrrogainous plant, the author proceeds to observe that 

 ibc mc lliod of feeding does not appear to be the same as in 

 in.ui.ili-., which pluck the plants they eat by means of the 

 two lobes above the upper jaw-pad, and push their food 

 towards the mouth with the tli|>pers. .Similar lobes cer- 

 tainlv cxi.st in the dugong, but they do not appear in 

 fresh s]iecimens to be ca[)able of any great degree of separa- 

 tion or movement, while the flippers are hardly long 

 enough to give any assistance in feeding. .-Xs the upper 

 jaw-p.id (u[)per lip) itself, on the other hand, is evidently 

 freely niov.ible, and possibly to some extent extensile, it 

 seems possible that it is used in plucking sea-weed, which 

 ccrt.-iinly could be grasped between it and the lower jaw. 



The Jiulbor adds that, according to the fishermen, a 

 single young one may be seeh with a female at any time 

 of the year; but on no occasion had they observed .a 

 female nursing its offspring with one of her flippers, while 

 lur head and fore-part of her body were raised out of the 

 w.iter after the fashion supjiosed to have given origin to 

 the mermaid myth. 



Colour Evolution in Monkeys. 



A p.apcr in ihr current issue of the Zool<igical Society's 

 I'liirtciliiHix is devoted to (be e\olution of a reniark.able 

 i\pe of colouring in the tropical .\frican monkeys com- 

 moulv known as guerezas, from the native name of an 

 .\bvssinian species. Starling from a wholly bl.ack species, 

 a gradual tr.insition can be Ir.aced to one in wliich the sides 

 of the f.ace, flanks, and hind-quarters, together with nearly 

 the whole of the tail, .are furnished with long fringes of 

 pure while hairs, a|>parently developed to accord with the 

 pendent white lichens clothing the branches of the boughs 

 among which these nionki ys ^lwell. 



Ox Warbles. 



.\ccording to Mr. .\. 1). I'urner, it is still unknown how 

 the m.itigois which form the tumours known as " warbles "' 

 on the b.icks of cattle ciTect an entrance into the bodies of 

 iheir hosts- whether by boring through the skin, or by being 

 sw.illowetl by the animals while in the egg-state, and subse- 

 qucntlv eating their way through the walls of the gullet, 

 ■ uid thus eventually reaching their final pl.ices of develop- 

 ment. 'l"hc point is an important one to cattle-owners, as 

 ■' warbles " are ,a source of very serious loss alike to the 

 IhiIcIui- and to the dealer in hiiies and leather. 



