49'-) 



NATURE 



[December 31, 1914 



weakened by heating -and the chemical affinity or the 

 amplitude of the molecular vibrations may be 

 strengthened by the energy of daylight. 



If we grant these assumptions, it is immediately 

 evident why the reaction of all dissolved colour sub- 

 stances of the first class is a neutral one, for the two 

 components may combine again and re-establish the 

 original substance. The other special qualities of the 

 first-class colours, and especially their differences from 

 the Giesel salts, which contain the electro-positive 

 component only, may be deduced likewise from this 

 retention of both components and their opportunity 

 of meeting each other again when the absorptive 

 power is weakened or the chemical afllnity is 

 strengthened. Now, the two components in the 

 coloured substances being distended in some degree, 

 I propose for this special condition of matter the name 

 of distension. If we accept this, have we created a 

 new name only, or does matter in this condition really 

 show new qualities? It seems to me that we have 

 to deal with a peculiar condition of matter, which 

 deserves a more elaborate study than it has met until 

 now. I will not enter again into some special quali- 

 ties, which have already been mentioned — the photo- 

 electric effect and so on — but I should like to point 

 out that matter in the distension state shov.'s a 

 strongly strengthened absorption of light. 



We noticed with regard to ammonium chloride the 

 yellow-greenish after-colour of the chlorine. Now, 

 kathode rays, as used in these experiments, will not 

 penetrate any deeper than one-hundredth of a milli- 

 metre into the salt. In such a thin layer even pure 

 liquefied chlorine would not show any perceptible 

 colour. But besides this it must be noticed that we 

 observe this after-colour at the temperature of liquid 

 air, and that chlorine at this temperature, as Dewar 

 and Moissan observed, is snow-white, even in thick 

 layers. In a similar degree the brown colour of 

 bromine is weakened at low temperatures. Now, if 

 nevertheless we observe at this very low temperature 

 the marked characteristic colours of chlorine and 

 bromine, we must conclude that the absorptive power 

 of these substances has become a multiple of its 

 ordinary value. One may observe this strengthening 

 of the absorptive power directly in the pure sulphur. 

 Sulphur likewise turns into a snow-white substance if 

 cooled by liquid air. But when the kathode rays fall 

 on the white sulphur it takes immediately a yellow- 

 reddish colour. It is a real after-colour, because at 

 constant low temperature the colour is destroyed by 

 daylight. 



Now, since the strengthening of light absorption 

 occurs in this elementary substance, it becomes evident 

 that the cause cannot be any chemical process, but 

 only a physical allotropy. The special character of 

 this allotropy (which may be connected with an absorp- 

 tion of electrons) will not be entered on in a discussion 

 here. Probably we have to deal with a polymerisa- 

 tion, so that, for instance, the yellow-reddish sulphur 

 would be analogous to polymerised oxygen — to ozone. 



I have mentioned already that the first-class after- 

 colours are gradually destroyed by incident daylight. 

 A peculiar phenomenon is connected with this destruc- 

 tion of colour. I found that after the daylight had 

 fallen on the coloured substances, even for the shortest 

 time, most of them showed a marked phosphorescence 

 of long duration. I have observed this phos- 

 phorescence even in substances which had been 

 coloured twelve years ago and had been kept in 

 the dark since that time. The diffused dim 

 light of a gloomy. November day, when falling 

 through a window on the coloured substance for one 

 or two seconds only, is sufficient for the production of 



NO. 2357, VOL. 94] 



this ' phosphorescence in a marked degree. If you 

 allow the daylight to fall several times on the same 

 spot, then the colour is weakened at this spot, and we 

 come to the presumption that the loss ot coloration 

 is generally attended by the emission of phosphorescent 

 light. This is in accordance with the experience of 

 Wiedemann and Schmidt that if the destruction of the 

 colour is produced by heating, likewise a phos- 

 phorescent light is produced, which in this case is 

 strong but of a short duration, corresponding to the 

 quick destruction of the after-colours by strong heat- 

 ing. 



If the salts, after having been coloured in the condi- 

 tion of a fine powder and then having been put be- 

 tween two glass plates (in order to obtain a plane 

 surface), are placed in a photographic camera instead 

 of the photographic plate, you may get a fine phos- 

 phorescent picture of a landscape or of architecture 

 after a very short exposure. 



Time does not allow me to mention in detail several 

 other peculiarities which are shown by matter in the 

 distension state. In one direction only I may be 

 allowed to make some remarks. 



The first-class after-colours may be produced not 

 only by kathode rays but also by the /3 rays of radio- 

 active substances, as you probably know But they 

 may also be produced by ultra-violet light, for in- 

 stance, by ultra-violet spark light, even when a quartz 

 plate is interposed between the spark and the salt. 

 More than thirty years ago I brought forward a hypo- 

 thesis, according to which in every point where 

 kathode rays strike a solid body a thin layer of ultra- 

 violet light-radiating molecules is produced in the gas, 

 to which ultra-violet light of very short wave-lengths, 

 for instance, the phosphorescence of the glass walls 

 in the kathode rays, is due. But I came further to 

 the assumption that nearly all effects which are com- 

 monly ascribed to special qualities of the kathode 

 rays, and likewise of ^ rays and X-rays, are mere 

 effects of the ultra-violet light which is produced by 

 the stopping of these rays. I have been guided by 

 this assumption during many years, and have very 

 often been aided by it in foreseeing new phenomena. 

 For instance, in this way I was induced to expect that 

 the after-colours would be produced not only by 

 kathode rays but also by the ordinary ultra-violet 

 light ; further I could guess that also the X-rays would 

 produce after-colours (which in this case have been 

 observed by Holzknecht), and in recent times I could 

 foresee that solid aromatic substances (the benzene 

 derivatives) in the ultra-violet light must change their 

 spectra of ordinary phosphorescence, composed of 

 broad bands, and turn to peculiar spectra composed of 

 narrow stripes, the wave-lengths of which are char- 

 acteristic of the single aromatic substances."^ So I 

 believe also that the after-colours are produced not 

 directly by the kathode rays or by j8 rays, but by the 

 aforesaid ultra-violet light which is connected with the 

 stopping of the other rays. 



In this way the after-colours enter at once into a 

 great class of phenomena known as reversible effects 

 of light. You know that certain effects of the visible 

 spectral rays are destroyed by rays of longer wave- 

 lengths, by the infra-red rays. And the analogy to 

 this phenomenon is in my opinion the destruction of 

 the after-colours : they are produced bv the ultra-violet 

 light of the stopped kathode rays and are annihilated 

 by the longer visible wave-lengths of daylight. In 

 this way you may likewise understand, for instance, 

 that the coloured spots, produced by X-rays on the 

 luminescent screens after long exposure, may be 

 destroyed again b}' exposure of the screens to day- 



6 £. Goldstein, Verhandl. d D. Pliysik. Ccs., xii. 



