356 



NATCRE 



[Al'cust 13, i8u6 



width of the second slit could be varied from O'l mm. up to 

 2 mm., and with the former width an exposure of fifty minutes 

 was recjuired. As long ago as March last, Dr. Fomm obtained 

 photographs showing interference bands, thus affording proof 

 of the undulatory nature of Ronlgen rays. By .starting with a 

 very narrow slit and gradually increasing its widdi, the inter- 

 ference lines approach closer together, until a dark line — 

 the first minimum — appears in the centre. .'Vs the opening be- 

 comes still wider, this minimum gives place to a maximum with 

 two minima, one at each side, and so on, and by means of Lom- 

 mel's formula, the wave-length can be determined from this 

 phenomenon. Dr. l'"omm obtains A = o 000014 mm., so that the 

 wave-length is about fifteen times smaller than the .smallest 

 wave-length hitherto observed in the ultra-violet. Owing to 

 the difficulty of determination, Dr. Fomm regards this number 

 as giving the upper limit rather than the exact measure of the 

 wave-length of the observed rays. Meanwhile MM. (;. Sagnac, 

 L. Calmette, and (_'.. T. Lhuillier have published investigations 

 in the same direction (Coinples rcmiiis, cxxii. 13 and 16). M. 

 Sagnac uses a wire grating, and from a scarcely measur- 

 able diftusion of the image of the slit he obtains 000004 as an 

 upper limit to the wave-length. MM. Calmette and Lhuillier 

 have made diftVaction experiments with two slits, and have ob- 

 tained bright and dark lines without expressing an opinion as to 

 the wave-length of the rays. 



Another closely allied question is whether Rcintgen rays con- 

 sist, like ordinary light, of radiations w-hose wave-lengths vary 

 over a considerable range. Such differences of wave-length 

 give rise in the case of light to the phenomenon of colour, and 

 the corresponding phenomenon for Rcintgen rays has been 

 studied by Dr. F.-V. Dwelshauvers-Dery {Bulh'tin de tAcadeiiiie 

 Royak de Be/ffii/iii', No 6) under the name of actinoihroisiii. Ob- 

 serving that differences in the degree of exhaustion of a Crookes' 

 tube might be expected to give rise to difl'erences of wave- length 

 in the emitted rays, and that the higher the vacuum the shorter 

 would the wave-lengths probably be, Dr. Dwelshauvers-Dery 

 has examined whether certain substances are more transparent 

 for certain Rbntgen rays than for others. For this purpose, 

 their transjjarencies were compared by placing the substances in 

 front of a fluorescent screen' and observing their shadows side 

 by side with that of a test-object consisting of laminii; of tin- 

 foil, whose total thickness could be varied at pleasure. To 

 obtain the necessary variation in the nature of the Rontgen 

 rays, it was found sufficient to compare the radiations from a 

 new tube, which had not been previously used, with those 

 emanating after the tube had been in action for some time. 

 The observations were repeated on the new tube after a quarter 

 of an hour, half an hour, an hour, an hour and a half, and two 

 hours respectively, and transparency-curves obtained by plotting 

 the results on paper. These curves show that ( i ) the trans- 

 parency of every specimen, with the exception of obsidian, 

 increases during the first few minutes ; (2) agate and alum, after 

 increasing in transparency for some time, become more and 

 more opaque ; (3) obsidian continually diminishes in trans- 

 parency. It is, of course, here a question of relative trans- 

 parency with respect to tin. Although we have no measure of 

 the variations of the absolute transparency of the tin itself, the 

 experiments suffice to prove that the absolute transparencies of 

 different substances vary according to the state of the tube, and 

 it is therefore, not considered hazardous to explain these 

 variations by the actinochroism of Rontgen rays. 



The same phenomenon has been observed by MM. Benoist and 

 Hurmuzcscu and, perhaps, by other physicists. In some of Mr. 

 A. A. C. Swinton's experiments it will be remembered that the 

 properties of Rontgen rays, and particularly their power of 

 penetrating through organic tissues, varied with the degree of 

 exhaustion of tlie vacuum. 



Two papers on Rontgen rays appear in a recent Bulletin 

 de tAcadhiiic Royale de Betgiqiie (No. 5). One, on the prob- 

 able cause of the production of Rbntgen rays and of atmospheric 

 electricity, and on the nature of electricity, is by P. de Heen. 

 Judging from the analogy of a pith ball oscillating between two 

 electrified plates, and from the comparative sizes of the pith ball 

 and the air molecule, it may be assumed that the molecules 

 have a velocity of 330,000 metres per second. This agrees fairly 

 well with J. J. Thomson's estimate of a velocity of 200,000 m. 

 per second for the kathodic projections Such a velocity corre- 

 sponds to the exces.sively high temperature of 46 million degrees. 

 Hence, wherever these molecules impinge upon a surface, they 

 w ill produce ether waves of very high frequency. These waves 



are probably identical with Ronlgen r.iy>, wliicli are llieicfore 

 very short ultra-violet waves. The author alvi claims to have 

 proved that an electrified surface impresses a sensitive plate quite 

 apart from any radiating action. He proposes the theory that 

 positive and negative electricity are projiagated in different ways, 

 the former by transverse, the latter by longitudinal, waves. 

 Atmospheric electricity is generated by mas.ses of gas emerging 

 from the interior of the sun (protuberances), which send out 

 ultra-violet waves, and charge the atmosphere positively and the 

 earth negatively by induction. 



The reflection of Rbntgen rays is treated by F. \'. Dwel- 

 shauvers-Dery, in the Bulletin referred to in the foregoing para- 

 graph. No trace of a regular or geometric reflection of Rbnfgen 

 rays can be discovered. The wave-length of the rays is 

 evidently loo small in comparison with the size of the mole- 

 cules. In order to find whether there was any diffuse reflection, 

 the author placed a sensitive plate with the film downwards. 

 A piece of ruby paper half covered the film, and sheets of 

 zinc, brass, copper, tin, and collodion were placed under this. 

 Then followed a second plate with the film upwards. On 

 exposing the whole to Rbntgen rays, both transmission and 

 reflection could be studied. As regards the former, it was found 

 that collodion increased the activity of the rays. This fact may 

 be utilised to diminish the exposure, a sheet of collodion being 

 placed above the object and the film. Impressions were also 

 obtained on the upper plate, apparently due to diffused reflec- 

 tion. The order of reflective power was : tin, zinc, copper, brass, 

 iron, platinum, gold, lead, aluminium. Hence tin placed below 

 the film may also be used to diminish exposure. The state of 

 polish of the surface was without influence, which shows that 

 there was no regular reflection. But the most important fact is 

 that the ruby paper intercepted a large proportion of the reflected 

 rays. Hence the latter are not Rbntgen rays proper, but rays 

 of greater wave-length, and it may be maintained that X-rays 

 are not reflected as such. 



Herr \V. Arnold ( CcnlralHatt fiir Nakriings- und Geiius- 

 smittel-Chemie sowie Hy^ene\ shows that Ronlgen rays can 

 be employed with considerable success in the detection 

 of food adulteration. Carbohydrates, fats, and aniline dyes 

 were found to be very transparent to lhe.se rays, though slight 

 differences were noticeable. Among the vegetable oils the order 

 of transparency was ; (i) castor oil, (2) almond oil, (3) olive oil 

 of Provence, (4) poppy oil, (5) oil of sesame, (6) linseed oil ; 

 the difierence between the last five was very slight, but castor 

 oil was considerably more transparent. Of fats, butter was the 

 least transparent, lard came next, and margarine was the most 

 transparent ; while the opacity of a mixture of different fats was 

 found to vary with the percentages of its constituents. Among 

 the spices, Herr Arnold found that the transparency decreased 

 as the proportion of ash increased, so that saffron was the least 

 and pepper the most absorbent of Rbntgen rays. F'oreign 

 matter mixed with spices, such as brick-dust, ochre, sand, &c., 

 was conspicuous, while adulterations of flour with powdered 

 fluor or other spar, or chalk, could readily be detected, liarthen- 

 ware glazes containing lead diftered strongly from ordinary 

 glazes, since, of all substances, lead offers the greatest resistance 

 to the passage of Rbntgen rays For colouring matters im- 

 bedded in gelatine the order was : (i) methylene blue, (2) 

 cyanin, (3) methyl violet, (4) eosin, (5) fuchsin, (6) brown, 

 (7) orange, (8) chrysanilin, (9) fluoresin ; the order must thus 

 be blue, red, yellow, .so that the lightest colours are the least 

 transparent. In wines the transparency decreased as the pro- 

 portion of sugar increased, just as generally the absorbing power 

 of fluids increased with their specific gravity, and that of the 

 elements with their atomic weight. In salts, the radical had 

 considerable influence, and arseniates, sulphates, and phos- 

 jihates exhibited a far greater power of absorbing the rays than 

 chlorides. 



The same writer also discusses the lumino.sity of solids under 

 the influence of Rbntgen rays. Referring to the use of fluor 

 spar in shortening the time of exposure of radiographs, as 

 employed by Winkelmann and others, Herr .\rnold states in the 

 Afothel;er-Z.e>luns\\vA he and Herr Forster-Bern have obtained 

 negative results, as no difference was noticed between the action 

 of the rays on plates exposed with and without the spar ; 

 possibly this was due to the quality of the spar employed. In 

 the Zcitsdirift fiir Electro-chemie he states the results of a long 

 series of observations on various forms of luminosity, namely, 

 ihermo-luminosity, kathodo-luminosity, and what he proposes to 

 call "X-luminosity." Herr Arnold finds tungstate of lime to 



NO. 1 398, VOL. 54] 



