INI AY 2 1, 1896] 



NA TURE 



53 



On the Action of Rontgen Rays and Ultra-violet Light 

 on Electric Sparks. 



In Nai cki-; uf April 30, the writer of " Recent Work with 

 Rontgen Rays" has not exactly described the results of our 

 experiments, [lublished in the KcndUoiili dell' Accadeiiiia del 

 Limei. 



We had formerly found that the sparking distance between 

 two electrodes, in a shunt -circuit on the discharge of an induction 

 coil, which illuminates a Crookes' tube, is strongly diminished 

 if tlie Rontgen rays sent from the tube fall upon the positive 

 electrode. The phenomenon is very interesting, as it is the 

 reverse of the phenomenon discovered by Hertz, in which the 

 ultia-violol light acts on the sparking distance in lengthening it, 

 when falMi)^ on the negative pole. 



On sul)se<jtient exjieriments, we found that when the sparking 

 distance was the same as that used with Rontgen rays, the ultra- 

 violet light acted exactly in the same way, and the passive pole 

 — so to say— was then the positive one. 



So far we had succeeded in reversing the phenomenon dis- 

 covered by Hertz, and further investigated by Wiedemann, 

 Ebert, Klster and (Jeitel, and had shown the parallelism of the 

 two radiations as to their impeditive action on the spark. 



But on diminishing the sparking distance, when the ultra- 

 violet light has a facilitating action, we have shown that the 

 Kiintgen rays would provoke the passing of the spark. In the 

 last case the jiassive pole — i.e. on which the radiation must fall 

 • — is in both cases the negative. 



So taking as electrodes two spheres of amalgamated brass, 

 52 mm. in diameter, when the sparking distance was below 

 30 mm., the R<)ntgen rays and the ultra-violet light provoke 

 the passing of the spark when falling on the negative electrode. 

 When, on the contrary, the distance was more than 30 mm. , 

 both radiations act in an impeditive way when falling on the 

 positive pole. 



This result is quite difVerent to that referred to in the cited 

 article, in which it is said that the simultaneous actions of the 

 Rontgen rays and tlie ultra-violet light could be made to 

 neutralise each other. Krom our experiments it follows, on the 

 contrary, that the action of the two radiations is in every respect 

 identical. 



•We will'describe a method by which the action of the Rontgen 

 rays on the spark is very clearly demonstrated. 



s, s' are the terminals of the secondary of an induction coil. 

 In front of the portion of the tube on which the kathode rays fall 



s a thin aluminium plate 20 cm. square, in connection with 

 the negative electrode of the Crookes' tube ; whilst the positive 

 is connected with a sphere so that the sparks between the plate 

 and ihe sphere take place in the direction of propagation of the 

 Rontgen rays, to which the aluminium plate is transjiarent. 

 The plate is connected to the earth. With this apparatus the 

 length of the spark can he made four times greater when the 

 Rontgen rays are screened before falling on the aluminium plate. 

 On diminishing the intensity of the current in the primary so as 

 to conveniently shorten the sparking distance, the inverse 

 phenomenon can be obtained, so that the spark does not pa.ss 

 «hen the R'lntgen rays are screened. 



NO. 1386, VOL. 54] 



Our present researches aim at the study of the alteration of 

 the nature of the discharge when the spark is under the action 

 of the Rontgen rays. A. Sella. 



Q. Majorana. 



Istituto fisico della Universita di Roma, May 3. 



Rontgen Ray Phenomena. 



.\ I' one of my demonstrations last week two tubes failed to act. 

 They were both of the kind which depend for their action on a 

 piece of |ilalinum placed within, and from which after bombard- 

 ment by kathode rays the Rontgen rays take origin. Aglanceat 

 the tubes showed that owing to the unusual strength of spark the 

 platinums within them were red-hot at the point of impact. 

 Before the demonstration the tubes had been in good worlcing 

 order. I considered they had broken down, but, on returning 

 home, tried them with the spark from my own apparatus, with 

 which they had before answered well. I was somewhat astonished 

 to find them giving off Rontgen rays rather more freely than when 

 first tried. This tends to show that Rontgen rays are not given 

 off by platinum heated above a certain temperature. I think 

 this has already been suggested, but I have not seen it cor- 

 roborated. 



Following up the idea of reinforcing the effect of the Rontgen 

 rays by placing a fluorescent screen under a sensitive film on 

 celluloid, the celluloid side being next the screen to prevent 

 "grain,'' and having tried screens of barium platino-cyanide, 

 potassium platino-cyanide, calcium tungstate, natural scheelite, 

 artificial scheelite (Edison's), fluor-spar and calcium fluoride, 

 I find that potassium platino-cyanide and artificial scheelite 

 alone produce any effect through celluloid. Barium platino- 

 cyanide, placed underneath, gave no effect either in contact 

 with the sensitive film itself or through celluloid, but the films 

 were not sensitive to yellow, and this salt gives yellow fluor- 

 escence. The effect with potassium platino-cyanide was decidedly 

 the best. 



Chard, May 3. J. William Gii-'I'ord. 



Alpengluhen. 



At' ii;k the shadows of the lower mountains have swept up 

 past the tops of the higher snow peaks, i.e. after the sun has 

 set upon these last, and as the general light of the sky fades, 

 the contrast between the illumination of the snow and of the 

 sky usually increases. The westerly-facing snow peaks stand 

 out against the darkened sky, and gradually change in tint. 

 Very often the most noticeable change is to a clear greenish- 

 white. But sometimes there is a period during which they have 

 a faint rose or crimson glow. This is the true Alpengluhen ; 

 often confused by tourists with the ordinary rose-coloured 

 illumination preceding the setting of the sun. 



I see (Nature, vol. liii. p. 58S) that it has been suggested that 

 this afterglow is due to what practically amounts to a second 

 rising of the sun upon the high snow, owing to a peculiar 

 arrangement of layers of hot and cold air in the atmosphere. 

 This may be so ; but it is rather a startling theory, and should 

 be tested by observations from, say, the higher or lower obser- 

 vatories of Mont Blanc, simultaneously with observations of 

 the Alpengluhen made from below anywhere to the west. A 

 reappearance of the sun would be an interesting sight for the 

 higher observer. In the meantime, my own observations of 

 some twelve or thirteen summers would lead me to suggest the 

 following explanation. In the first place, I do not think that 

 the afterglow is nearly as vivid as an observer believes. To the 

 eye, the stars " come out," and the moon becomes aknost 

 dazzling, as the general light of the sky fades ; and both " fade " 

 as day breaks. 



Next, I noticed the following during five months of uninter- 

 rupted observations of sunsets in the plains of Argentina. On some 

 fine evenings, there was left, as daylight faded, a vivid line oy 

 band (of uneven thickness) of intense crimson colour in the 

 west. This was so strong and so well defined that it lit up the 

 westerly face of the estancia with crimson, and actually threw a 

 fairly sharp shadow of the horizontal gutter. Vertical poles, 

 iVc., had of course no shadow ; the source was too long in a 

 horizontal direction. This crimson streak did not appear 

 always, by any means. The westerly sky itself often passed 

 through various tints of a clear greeni.sh-blue. 



It seems to me that, considering the snow heights facing the 



