594 



NATURE 



[January 30, 191- 



The Reflection of the X-Rays. 



Messrs. Laue, Friedrich, and Knipping's remark- 

 able photographs taken through a crystal with X-rays 

 have opened a new field of research. Mr. VV. L. 

 Bragg- has shown that much stronger photographs 

 are produced by a grazing reflection from mica. We 

 have recently used the latter discovery to study the 

 reflected beam electrically. We find that it resembles 

 ordinary X-rays. Just like tlie primary rays it ionises 

 air and helium and produces a soft radiation when 

 it strikes metals. The variation of its ionisation in 

 air with pressure is also similar. We have so far 

 obtained effects as great as i or 2 per cent, of the 

 primary beam. In some cases the measurements in 

 helium were magnified by ionisation by collision. 



The same absorption of the reflected beam was 

 produced bv aluminium, whether the rays passed 

 through it before or after the reflection. In one case 

 the absorption coefficient for either position was 

 6 cm.-i, whereas for the primary beam it was 

 9 cm.-^ This indicates that the character of the 

 rays is unaltered by reflection, but that the amount 

 of reflection increases with the hardness. 



As Prof. Bragg has shown, the behaviour of the 

 X-rays in connection with ionisation strongly suggests 

 that their energy is concentrated as if they were 

 corpuscular. Since the rays are reflected, they must 

 be some kind of pulse with an extended wave-front, 

 yet after reflection they retain their corpuscular char- 

 acter. Thus the energy of the X-rays appears to show 

 the contrary properties of extension over a wave-front 

 and concentration in a point. H. Moseley. 



C. G. Darwin. 



Victoria University, Manchester, January 21. 



Emission of Particles by Heated Metals. 



Referring to Prof. Valenliner's article on "New 

 Facts in Physics," in Nature of January 9, you note 

 that Messrs. Reboul and Dr. Bollement have found 

 that copper and silver eject particles at 500° C. in a 

 vacuum, in air, oxygen, and carbonic acid, which 

 forms deposits on the walls of the containing vessel. 

 Pure silver, and several alloys having a chief content 

 of silver, were found by me to emit particles or an 

 emanation of some sort when heated in air to a 

 bright red, but well below their melting points. The 

 facts were observed during an inquiry to ascertain the 

 cause of the sudden failure of the platinum pins of 

 porcelain teeth, and were briefly noted in The Dental 

 Record, September, 191 1. 



In the presence or proximity of silver at a red 

 heat two kinds of effects on other bodies were 

 observed: (i) a yellow to brownish-yellow staining on 

 the surface of various porous "investments" (plaster 

 of paris, pumice-stone, whiting, French chalk, 

 asbestos, and mixtures of these) against which the 

 silver or silver alloy had been heated ; (2) porcelain 

 was rendered weak and almost friable to a varying 

 depth from the surface in contact with silver, the 

 deteriorating effect penetrating to a depth of 3 or 

 4 mm. during half an hour's heating. 

 _ Gold, platinum, copper, zinc — singly or in combina- 

 tion — did not produce those effects, but when silver 

 was present the staining was copious, and the weaken- 

 ing or "fritting" of porcelain in at least some degree 

 always resulted. No analysis was made, but the 

 inference seemed well founded that in those cases 

 silver at a bright-red heat emitted something that 

 gave rise to the effects noted- D. M. Shaw. 



Eltham. S.E 



Thermal Efficiency of Gas and Electricity. 



Our attention has been directed to a short para- 

 graph in your issue of Jnnu.-n-y 16 (p. 551) regarding 

 NO. 2257, VOL. 90] 



the progress of electric cooking, in whicli the follow- 

 ing sentence occurs: — "The present year should see 

 a reasonably cheap and economical electric oven put 

 on the market to compete with the everyday gas- 

 cooker, which at present, on account of its low initial 

 cost, still holds the field against the electric oven 

 among the general public." 



Please allow me to point out that it is not inerely 

 a question of comparative cost and upkeep of appa- 

 ratus, but the more important point of comparative 

 efliciency. The heat equivalent of one unit of elec- 

 tricity (costing at least one penny) equals 3410 British 

 thermal units, while the heat equivalent of 33 cubic 

 feet of gas (costing in London one penny) equals 

 16,500 British thermal units. No science can obtain 

 from a given quantity of energy or fuel more than the 

 maximum amount of heat it produces when completely 

 consumed, and it is therefore apparent that electricity, 

 so far as heat production is concerned, can never 

 economicallv compare with gas. W. M. Mason, 



Secretary. 



The British Commercial Gas Association, 



47 Victoria Street, W'estminster, S.W., 

 januarv 21. 



Research Defence Society. 



It is said that the fiftli year in the life of any 

 society is the critical period of its fortunes. The 

 Research Defence Society was founded on January 27, 

 1908. To all who are interested — and who is not? — 

 in medical research, we beg you to let us say that 

 the society has its hands full of work, and only wants 

 more money to do more work. Much has already 

 been done, by lectures and by distribution of litera- 

 ture, to bring home to people the truth about experi- 

 ments on animals in this country, and the great value 

 of them, not only to mankind, but also to the animal 

 world. The expenses of our society are heavy; but 

 the good results of our work are extended far and 

 wide. We have lately opened a bureau and e-xhibition 

 at 171 Piccadilly (opposite Burlington House). We 

 are exhibiting pictures, portraits, charts, ansesthetics 

 and inhalers, germs in pure culture, tsetse-flies and 

 mosquitoes, and so forth. This little exhibition, every 

 day, and all dav long, displays to "the man in the 

 street" the facts of the case. We are the only societ)' 

 which is doing work of this kind ; but, of course, it 

 cannot be done without monej'. Our record for the 

 last four years gives us the right to hope for a great 

 increase of our membership, and of our funds, in the 

 coming year. David Gill, President. 



F. M. Sandwith, Hon. Treasurer. 



Stephen P.aget, Hon. Secretary. 

 21 Ladbrolce Square, W., January 24. 



Retinal Shadows? 



If, in the early morning, when the eyes are first 

 opened, one, looks at the white ceiling, branching lines 

 are to be seen resembling blood-vessels. These figures 

 only persist for about one second. It seems neces- 

 sary for the eyes to be closed, and have a long rest, to 

 show them clearly. In the daytime quite a long rest 

 is necessary for them to be visible at all. 



So far as I can judge they always have the same 

 form. It would appear from this that they are really 

 the shadows of the blood-vessels ; but why they should 

 onlv be visible for about a second after opening the eyes, 

 and then only after a long rest, does not seem clear. 



It may be that the phenomenon is well known, but, 

 if so, I have not chanced to hear of it. 



R. M. Deeley. 



.\bbeyfifld, Salisbury Avenue, Harpenden, 

 January 21. 



