NA JURE 



[June 4, 1896 



It follows from these experiments of Haga's, that Rcintgen rays 

 act on the resistance of selenium in the same way as light and 

 heat ravs do. I think selenium will be found to be very useful 

 in investigating the opacity of diflerent substances for the 

 Riintgen rays, and also for experimental work on the polarisa- 

 tion of those rays, as the deflection of a galvanometer is much 

 easier to appreciate tlian the value of the photochemical action 

 of the rays. It also follows from these experiments, that selenium 

 is a very unfit material for making photometers. 



It must always be kept in mind, when working with selenium, 

 that the cell takes \ery little time in diminishing its resistance 

 under the action of light, or other rays, but that it tsikes a much 

 longer time (often half an hour or so) to rettirn'to its state of 

 high resistance. It follows from this, that if one wishes to com- 

 pare the action of two emissions, one must begin with the 

 feebler radiation and afterwards let the stronger radiation hit 

 the cell. 



The Nature of Kiiittgen Rays. 



The nature of Riintgen rays is so far from being settled, that 

 the following remarks by Prof. W. N. Hartley, in favour of the 

 ultra-violet theory, will be read with interest : — 



The great doubt which prevails as to the nature of Rdntgen 

 rays arises from the fact that it is difficult to imagine radiations 

 which make their existence manifest in a manner which, at first 

 sight, appears very extraordinary. 



Their intractability to the action of ordinary refractive media, 

 and the facility with which they are transmitted by matter 

 which has the property of absorbing light, has led to their being 

 regarded as being propagated by vibrations differing in direc- 

 tion from those of other known forms of energy. I have given 

 expression to the view on more than one occasion that they are 

 simply ultra-violet radiations of much greater oscillation fre- 

 quency than any we have yet been able to recognise and manipu- 

 late with prisms and lenses. The following comparison of the 

 properties of the ultra-violet with those of the Rontgen rays are 

 my reasons for this view : — 



(1) Ultra-violet rays can be reflected and refracted. 



(2) They are capable of energetic chemical action. 



(3) They cause fluorescence. 



(4) They facilitate the discharge of electricity through air. 



(1) The Rontgen rays can be reflected, but have not hitherto 

 been refracted. 



(2) They cause energetic chemical action. 



(3) They cause fluorescence and also phosphoresence. 



(4) They cause the discharge of electricity through a non- 

 conductor. 



The ultra-violet rays are also subject to energetic absorption, 

 which increases with the molecular mass of the absorbing sub- 

 stance in certain cases, but is dependent upon molecular structure 

 in other instances. Rontgen rays are also absorbed energetic- 

 ally by some substances, and the absorption appears to be de- 

 pendent upon the molecular mass of the absorbing medium, 

 but in other cases it appears to depend upon what is probably 

 molecular structure. Both the ultra-violet and Rontgen rays 

 are revealed to us by their action on a photographic plate and 

 on fluorescent substances, or rather substances which they render 

 fluorescent. 



The ultra-violet rays e.xcite fluorescence in almost all sub- 

 stances, and with very remarkable effects in many cases (_/. 

 Chem. Soc, vol. clxiii. p. 247, 1S93). The effect of substances 

 on the rays which enter them is to retard their rate of vibration. 

 By retardation the length of the waves is increa.sed to dimen- 

 sions which bring them within the limits of visibility, and the 

 result is either fluorescence or phosphorescence. This is usually 

 expressed by saying the rays are lowered in refrangibility. It 

 is quite ])robable that we may soon have evidence of refraction 

 of Rontgen rays. They are undoubtedly reflected, since Jackson 

 has shown that the most effective form of Crookes' tube is one 

 in which a plate of platinum at an angle of 45° reflects the rays 

 through the side of the vessel (Proc. Chem. Soc, March 6, 

 1896). 



Rontgen rays are in all probability of the same character as 

 the ultra-violet rays ; they produce the same effects, and no 

 other rays are known to do this, except such as are of the same 

 character and are capable of being "lowered in refrangibility," 

 or retarded But it is evident that they must be of much greater 

 oscillation frequency, or what amounts to the same thing, of 



much shorter wave-length than any which have hitherto been 

 studied with lenses and prisms of rock-crystal or fluor-spar. 



Mr. Jackson has declared his adherence to the l)elief that 

 they are propagated by transverse, and not by longitudinal, 

 vibrations (/. Chem. Soe., vol. clxv. p. 734, 1894: also Proc. 

 Chem. Soc., March 6, 1896). 



I have been induced to place these remarks on record, because 

 in Naturic (p. 45) there appears an alKtract of a paper in 

 Wiedemann's Annalen, by U. A. Goldhammer, which renders 

 it evident that from other considerations he is of the same 

 opinion. He points out that the peculiarities of Rontgen rays 

 are not inconsistent with transverse vibrations of very small 

 wave-length. His reference to the aljsence of reflection appears 

 to be not strictly accurate, so far as one may judge from the 

 words of the abstract ; but it is not fair to draw conclusions 

 from an author's views without regard to his ipsissima verba. 



Analysis of A'onlgen A'ays. 

 Mr. T. C. Porter, of Eton College, appears to have made 

 an interesting discovery in connection with the Rontgen rays, 

 viz. that they are of at least two difierent kinds. We print in 

 full a preliminary account of the experiments which have led to 

 this conclusion, with the remark that the photographs received 

 just as we go to press entirely bear out the description. 



A Rontgen tube (Newton and Co.'s and Griflin's focus tubes 

 have been u.sed in these experiments) emits two diflerent kinds 

 of rays. To one kind, which I venture to call Xj, flesh is fairly 

 transparent, and hone opaque ; to the rays of which this is a 

 preliminary account, which will be called hereafter X„, flesh 

 seems nearly, if not quite as opaque as bone. Under ordinary- 

 circumstances, in the cold, using an induction coil (3V' spark) 

 and somewhat rapid hammer contact breaker, most, but not all, 

 of the rays are X, ; but if the tube lie heated, less and less of X, 

 are emitted and more of Xj until the fluorescent screen (mine is 

 one of Messis. Reynoldsand Branson's, of Leeds, bright yellowish 

 green in colour, and apparently of uranium glass, though of 

 this I am not sure) shows the shadow of a hand held behind it 

 sharply defined and very dark all over, the bones not being 

 visible. The back of the screen is covered with a layer of very 

 opaque (to ordinary light) thick black paper. Up to a certain 

 temperature the green fluorescence of the glass of the tube 

 increases very markedly, but the X, rays do not come from it, 

 as the sharpness of the shadow shows ; nor are the Xo rays 

 ordinary kathode rays, for the same discharge sent through a 

 highly exhausted Crookes' tube showing "independence" ot 

 the positive pole failed to excite any fluorescence whatever on 

 the screen, though the glass of the tube was fluorescing 

 Ijrilliantly opposite the concave kathode, and the violet cone of 

 rays within the tube was plainly visible. At a certain tempera- 

 ture, judging from the fluorescence on the screen, the emi.ssion 

 of these X, rays reaches a maxinnmt, and on further heating 

 the emission of any rays whatever capable of exciting fluor- 

 escence or photographic action falls oft' rapidly, though, so far as 

 my experiments have gone, some fluorescence and photographic 

 action have been plain up to the highest temperature to which 

 I judged it wise to heat the tube. Wood and paper seem very 

 fairly transparent to the X.^ rays, but glass seems very opaque, 

 aluminium much more opaque than to the Xj rays, judging by the 

 following experiment, which shows best the existence of these 

 radiations and their difference from the Xj radiations. 



A " Rontgen" whole plate was wrapped in two thicknesses 

 of the black paper generally used for the purpose, and supplied 

 with the plates by the Sandell Plate Co., and brought in dark- 

 ness into the room for experiment, lit dimly by a single candle 

 at some distance from the place where the plate was to lie. 

 The plate was then laid film uppermost (still, of course, wrapped 

 in the black paper) six inches below the exhausted tube (the 

 latter placed in the usual position). A piece of plate-glass, one- 

 third of an inch thick, was then laid over half of it, and a left 

 hand laid on the other half, together with a piece of a small 

 aluminium tray, and exposure was made for one minute with 

 the exhausted tube cold (16° C). The paper over the exposed 

 half was then marked for the purpose of recognition ; this half 

 was then covered with the glass, to protect it from any further 

 action, and the photographic plate turned in its own plane 

 through 180° about a vertical axis, to enable the operator to 

 place his hand on the other half in exactly the same way as at 

 first. The tube was then heated with a spirit-lamp giving a large 

 flame for about forty-five seconds, and, the left hand being in 



NO. 1388, VOL. 54] 



