April 2, 1896] 



NATURE 



523 



that of the applied counter potential, but is of opposite sign. 

 The plates were of zinc and tinfoil, the latter being mounted on 

 thin ebonite to keep it flat. They were placed parallel to one 

 another at a small distance apart, so that the rays fell perpen- 

 dicularly on the back of the tinfoil plate, passed through it and the 

 air space between them, and were absorbed by the zinc. The 

 tinfoil plate was insulated and connected to the insulated quad- 

 rants of a Kelvin quadrant electrometer. The zinc was uninsu- 

 lated, and was connected to the uninsulated quadrants of the 

 electrometer. This plate was movable in a direction perpen- 

 dicular to its plane, and could thus be drawn away from the tin- 

 foil. If there was any electric potential difference between the 

 opposing surfaces of the two plates, further separation caused a 

 change in it which, reacting on the electrometer, deflected it. 



Cathode 



Plan 



Tt 



Seals \ 



Fig. 2.— a, vacuum tube ; nn', lead plate, i8 in. x 



hole 2 in. diameter. Pipes 2 in. diameter x J in. ; d, speculum-met 

 mirrors ; e, zinc box, 4 m. X 4 in., i in. deep ; i-, aluminium window 

 G, objects ; h, black cardboard ; i, photographic plate. 



i in. thick ; c, 

 ;al 



Experiments conducted with this arrangement led Mr. 

 Murray to conclude that (l) the influence of the rays on the 

 zinc and tinfoil plates does not cause any direct or sudden change 

 in their contact potential, but that (2) the air through which the 

 rays pass is temporarily converted into an electrolyte, and when 

 in this condition forms a connection between the plates which 

 has the same properties as a drop of acidulated water, namely, 

 it rapidly reduces the potential between the opposing surfaces of 

 the plates to zero, and may even reverse it to a small extent. 



It is pointed out that this electrolytic property was found by 

 Lord Kelvin (" Electrostatics and Magnetism," Art. xxiii,§§4i2 

 —414) to be possessed by the fumes from a burning spirit lamp. 



NO. I 379. VOL. 53] 



In both cases its cause is probably the same. It is, no doubt, 

 due to a want of electrical equilibrium among, and a partial 

 dissociation of, the molecules of the gas. 



In the third of the Royal Society papers referred to in the 

 foregoing, Mr. C. T. R. Wilson gave an account of experiments 

 made to determine the effect of Rontgen rays on cloudy con- 

 densation. The author has previously shown that cloudy 

 condensation takes place in the absence of du-st when saturated 

 air suffers sudden expansion exceeding a certain critical amount. 

 He now finds that air exposed to the action of Rontgen rays 

 requires to be expanded just as much as ordinary air in order 

 that condensation may take place, but these rays have the effect 

 of greatly increasing the number of drops formed when the 

 expansion is beyond that necessary to produce condensation. It 

 is concluded that when the Rontgen rays pass through moist air 

 they produce a supply of nuclei of the same kind as those which 

 are always present in small numbers, or at any rate of exactly 

 equal efficiency in promoting condensation. 



At the Royal Dublin Society, on March 18, Mr. Richard 

 J. Moss read a paper on the Rontgen X-rays. The mixed 

 gases obtained by the electrolysis of hydrochloric acid were 

 submitted to the action of X-rays. The apparatus employed 

 was siinilar to that devised by Bunsen and Roscoe for their 

 "Photo-Chemical Researches" (Phil. Trans., vol. cccxlvii. 

 P- 355)- Every precaution was taken to ensure that the 

 hydrogen and chlorine were in atomic proportions, and free from 

 impurity. It was found that the X-rays, at a distance of 5 

 centimetres from the Crookes' tube, caused no combination of 

 the hydrogen and chlorine. The combination of o'l per cent, 

 of the volume of gas operated upon could not have escaped 

 detection. The X-rays from the Crookes' tube employed were 

 sufficient to produce fluorescence in a crystal of lithium rubidium 

 platinocyanide at a distance of 2 metres. The examination of 

 a series of crystalline platinocyanidcs showed that lithium 

 rubidium platinocyanide emitted the brightest light under the 

 influence of the X-rays. The same order of luminosity wa& 

 observed with ultraviolet light as with X-rays ; and the colour 

 of the fluorescence is similar whether excited by X-rays or by 

 ultra-violet. Crystals of platinocyanidcs which exhibited yellow 

 and green fluorescence under the influence of the X-rays» 

 behave as bodies opaque to those rays when photographed. 

 Potassium platinocyanide exhibits blue fluorescence, and if a 

 crystal of this salt, separated from the sensitive plate by a sheet 

 of white paper, be submitted to the action of the X-rays, it 

 behaves as a luminous body and does not throw a shadow. The 

 fluorescent light excited by the X-rays in this salt, acts more 

 powerfully upon the sensitive plate than the direct X rays. 



Dr. John Macintyre has .sent us an account of a further study 

 and comparison of fluorescent screens. He says : 



" I have tried several screens of calcium tungstate as recom- 

 mended by Mr. Edison. The insoluble amorphous precipitate 

 is fluorescent, but I have had it prepared in the crystalline form 

 from two different sources ; in both cases the screens gave much 

 better fluorescence, and consequently better images. Messrs. 

 Baird and Tatlock prepared the one form of cry.stalline salt, 

 which was larger in the crystals than that prepared for me by 

 Mr. I. Frank Bottomley. As there seems to be some difficulty 

 in obtaining the crystalline form, it may be of interest to say tha 

 the latter gentleman prepared it according to the method of N. 

 S. Manross ("Liebig's Annalen," vols. 81-82). The method 

 consisted in fusing together the anhydrous sodium tungstate and 

 calcium chloride, the latter in excess The sodium chloride and 

 excess of calcium chloride are dissolved out, and crystalline 

 calcium tungstate is left behind to be filtered off and dried. 

 The crystals are a mixture of needles and octahedra, the latter 

 only recognisable through the microscope. 



" As far as I have been able to judge from the preparations 

 given to me, potassiumplatino-cyanide is the best salt. Of 

 course, Mr. Edison may have a better method of preparing the 

 crystals. With greater experience in screens, I am inclined 

 to' think that success depends very largely upon the way in 

 which they are prepared. The paper or glass covering must be 

 thickly and evenly coated. I have had an excellent screen 

 prepared with the barium salt as well. 



" My experiments have mainly been with the human skeleton ; 

 I have now seen shadows of all the bones and joints of the 

 extremities, and what is more important, by placing the tube in 

 the proper position I have seen distinct shadows of the spinal 

 column and ribs of a man. The head is easily penetrated, and 

 for surgical purposes all we now require in order to obtain greater 



