June 1, 189G.] 



KNOWLEDGE, 



121 



LONDON: JUNE 1, 1896. 



CONTENTS. 



^ PAGE 



The Nature oftlie X Rays of Rontgen. By J. J. Siewakt, 



B.A.Cantab., B.Sc.Lond " 121 



Brief Description of the Orchid Photographs. By 



II. A. ISruBEKiiV, F.K.H.S. {Illustrated) 122 



A Geographical Description of the British Islands. 



By lIi-Gii RoiEiT Mill, ])Si'., F.R.S.E 123 



Protective Resemblance in the Nests and Eggs of 



Birds. Bv Haery F. Witueuby. {Illustrateil) ... 12.j 

 Sun-Symbols' in Ancient Egypt. By F. W. Rkad 



{Illustrated) 127 



The Royal Society of Painters In Water Colours ... 128 



Science Notes 129 



Letters :— Dayid Flaxeey \ Dayid E. IIadde.v ; IIelios ; 



V. II. Wohslkt-Bexisox 130 



The Approaching Total Eclipse of the Sun. By A. 



FoWLEn, F.R.-\.S. [[lluxiraled) ' ... 131 



Photograph of the Cluster Messier 24 Clypel. By 



Isaac ROBEKT.^, D.Sc, F.R.S. (Plate) ". 134 



Notices of Books ... 135 



Waves.— VI. Standing VJaMe.% in Flowing Water. By 



Vatjohan CoRNlsn, M.Sc. (Illu.^trated) ... ... ... 136 



A Rare Metal. By T. L. PiiirsoN. Ph D 140 



Sertularlan Polypldoms; or •'Horny Corallines." By 



P. L. Addison, F.Ct.S., Assoc.M. lust.C.E. {Illustrated) 141 

 The Face of the Sky for June. By Hebbebt Sadleb, 



F.R.A.S 143 



Chess Column. By C. D. Xooock, B_A..Oion 113 



THE NATURE OF THE X RAYS OF RONTGEN. 



By J. J. Stewart, B.A.Cantab., B.Sc.Lond. 



PROBABLY at uo period in the history of science 

 has so much research been concentrated on one 

 particular part of the field of investigation as 

 now, when in all the principal physical labora- 

 tories of Europe inquiries are being made into the 

 effects and the probable causes of the remarkable new 

 species of radiation labelled by one of the early discoverers 

 X, or unknown. To able and well-directed efforts thus 

 persistently made, " the rays " will no doubt soon be 

 obliged to yield the secret of their origin and cause. In 

 fact, already there are clear indications of the direction in 

 which the truth may be discovered, and the researches of 

 explorers in several widely separated countries are con- 

 verging on one definite explanation of the source of this 

 mysterious phenomenon. Opinions, however, amongst 

 those competent to judge are still in many quarters 

 widely different. 



Recently amongst our neighbours the French, who have 

 been active in physical research, a form of radiation has 

 been brought to notice which differs from ordinary light 

 and is different also from the rays described by Rontgen. 

 This newly-found form of ethereal vibration appears to 

 possess characteristics intermediate between those of light 

 which affects our eyes and those associated with the X 

 rays. Most people have seen in the shop windows, or 

 elsewhere, yellow or canary-coloured glass, sometimes 



made into small trays or dishes. It possesses a peculiar 

 fluorescent property, and when light of any colour what- 

 ever falls upon it, it glows with its own peculiar and weird 

 phosphorescent yellowish light. This property of the glass 

 is remarkably shown by placing a rod made from it in a 

 dark room and forming a spectrum of some source of 

 white light. When the rod is placed in the red or the blue 

 part of the spectrum, it still persists in giving back its 

 characteristic yellowish shimmer. The cause of this 

 behaviour in the glass is the presence in it of salts of the 

 somewhat rare metal uranium. Now, il. Henri Becquerel, 

 in France, has been carrying out investigations on 

 the nature of the radiation given out by various salts 

 of uranium, and has shown the existence of radiation 

 intermediate in character between that of light and that 

 which probably causes the Rontgen effects. M. Becquerel 

 made a fluorescent screen of thin crystals of the double 

 sulphate of uranium and potassium, a substance which 

 is very active in its phosphorescence. This phosphores- 

 cence or visible emission of light lasts a very short time, less 

 than one-hundredth of a second: but the fluorescent screen 

 appears to have also the power of giving forth invisible phos- 

 phorescent radiations. Light from the sun after passing 

 through such a screen can cause the formation of an 

 image on a sensitive film, although black paper or alumi- 

 nium is interposed in the path of the solar radiations. 

 Copper is, however, almost opaque to these radiations. 

 Moreover, a film left in the dark, with the phosphorescent 

 screen almost touching it, shows intense images. These 

 latter are not produced by any visible light, and must be due 

 to the invisible phosphorescence. The persistence of the 

 emission of these dark radiations from the uranium salt is 

 most extraordinary. After being kept in the dark for a fort- 

 night the screens formed from the salt emit their radiations 

 with scarcely less intensity than when freshly exposed to 

 light. These uranium radiations discharge electrified 

 bodies just as Rontgen rays do : they also suffer the same 

 sort of diminution in intensity 1=! the rays of RiJntgen on 

 traversing a plate of quartz. 'heir action, however, has 

 only about one-hundredth par^ cl the intensity of that of 

 the Rimtgen rays. Various s^Us of uranium exhibit the 

 same properties. 



The special interest attacl:ing to these experiments at 

 present is due to the fact that ^1. I'.ecquerel finds that the 

 uranium radiations can be refracted and polarized. Now, 

 this polarization proves that tlie radiation must consist of 

 transrei-.-ie vibrations ; and as Rijutgen radiation is similar in 

 many respects, a presumption arises that the Rilntgen rays 

 themselves are caused by transverse vibrations in the 

 ether. Recent work shows distinct evidence that the 

 Rilntgen rays can be reflected. These rays, then, possess 

 some of the properties of ordinary light. 



An Italian investigator. Signer Righi, finds that when a 

 body charged with negative tlcctricity is exposed to the 

 action of RiJntgen rays it loses its negative charge, which 

 appears to leak away ; and when longer exposed to the 

 same influence it gains a positive charge instead. Invisible 

 rays from the part of the spectrum beyond the violet 

 exert a similar action, and there is thus shown to exist 

 another property common to those two sorts of radiation. 



Other observers have found tluit the light given out by 

 phosphorescent bodies is able to penetrate substances which 

 are quite opaque to ordinary light from the sun. Oifliisc 

 reflection of Rontgen rays seems to have been distinctly 

 observed, and images of opaque objects have been produced 

 by means of reflected rays when the sensitive plate on 

 which the images were formed was screened from tho 

 direct action of tho vacuum tube by means of a thick 

 sheet of copper placed between the tube and tho plate. 



