446 



NATURE 



[September 8, 1898 



phosphorescent properties of each of these groups are profoundly 

 modified by an admixture of foreign bodies — witness the effect 

 on the lines in the phosphorescent spectrum of yttrium and 

 samarium produced by traces of calcium or lead. The per- 

 sistence of the samarium spectrum in presence of overwhelming 

 quantities of other metals, is almost unexampled in spectroscopy : 

 thus one part of samaria can easily be seen when mixed with 

 three million parts of lime. 



Without stating it as a general rule, it seems as if with a non- 

 ' phosphorescing target the energy of molecular impact reappears 

 as pulses so abrupt and irregular that, when resolved, they 

 furnish a copious supply of waves of excessively short wave- 

 length, in fact, the now well-known Rontgen rays. The phos- 

 phorescence so excited may last only a small fraction of a 

 second, as with the constituents of yttria, where the duration of 

 the different lines varies between the 0*003 ^"^ '^^ 0*0009 

 second ; or. it may linger for hours, as in the case of some of the 

 yttria earths, and especially with the earthy sulphides, where 

 the glow lasts bright enough to be commercially useful. Ex- 

 cessively phosphorescent bodies can be excited by light waves, 

 but most of them require the stimulus of electrical excitement. 



It now appears that some bodies, even without special stimu- 

 lation, are capable of giving out rays closely allied, if not in 

 some cases identical, with those of Prof. Rontgen. Uranium 

 and thorium compounds are of this character, and it would 

 almost seem from the important researches of Dr. Russell that 

 this ray-emitting power may be a general property of matter, 

 for he has shown that nearly every substance is capable of 

 affecting the photographic plate if exposed in darkness for 

 sufificient time. 



No other source for Rontgen rays but the Crookes tube has 

 yet been discovered, but rays of kindred sorts are recognised. 

 The Becquerel rays, emitted by uranium and its compounds, 

 have now found their companions in rays— discovered almost 

 simultaneously by Curie and Schmidt — emitted by thorium and 

 its compounds. The thorium rays affect photographic plates 

 through screens of paper or aluminium, and are absorbed by 

 metals and other dense bodies. They ionise the air, making it 

 an electrical conductor ; and they can be refracted and probably 

 reflected, at least diffusively. Unlike uranium rays, they are 

 not polarised by transmission through tourmaline, therefore 

 resembling in this respect the Rontgen rays. 



Quite recently M. and Mdme. Curie have announced a dis- 

 covery which, if confirmed, cannot fail to assist the investigation 

 of this obscure branch of physics. They have brought to notice 

 a new constituent of the uranium mineral pitchblende, which in 

 a 400-fold degree possesses uranium's mysterious power of 

 emitting a form of energy capable of impressing a photographic 

 plate and of discharging electricity by rendering air a conductor. 

 It also appears that the radiant activity of the new body, to 

 which the discoverers have given the name of Polonium, needs 

 neither the excitation of light nor the stimulus of electricity ; 

 like uranium, it draws its energy from some constantly re- 

 generating and hitherto unsuspected store, exhaustless in 

 amount. 



It has long been to me a haunting problem how to reconcile 

 this apparently boundless outpour of energy with accepted 

 canons. But as Dr. Johnstone Stoney reminds me, the re- 

 sources of molecular movements are far from exhausted. There 

 are many stores of energy in nature that may be drawn on by 

 properly constituted bodies without very obvious cause. Some 

 time since I drew attention to the enormous amount of locked up 

 energy in the ether ; nearer our experimental grasp are the 

 motions of the atoms and molecules, and it is not difficult 

 mentally so to modify Maxwell's demons as to reduce them to 

 the level of an inflexible law, and thus bring them within the 

 ken of a philosopher in search of a new tool. It is possible to 

 conceive a target capable of mechanically sifting from the 

 molecules of the surrounding air the quick from the slow 

 movers. This sifting of the swift moving molecules is effected 

 in liquids whenever they evaporate, and in the case of the 

 constituents of the atmosphere, wherever it contains constituents 

 light enough to drift away molecule by molecule. In my mind's 

 eye I see such a target as a piece of metal cooler than the 

 surrounding air acquiring the energy that gradually raises its 

 temperature from the outstanding effect of all its encounters 

 with the molecules of the air about it ; I see another target of 

 such a structure that it throws off the slow moving molecules 

 with little exchange of energy, but is so influenced by the quick 



NO. 1506, VOL. 58] 



moving missiles that it appropriates to itself sonie of their 

 energy. Let uranium or polonium, bodies of densest atoms, 

 have a structure that enables them to throw off the slow moving 

 molecules of the atmosphere, while the quick moving molecules, 

 smashing on to the surface, have their energy reduced and that 

 of the target correspondingly increased. The energy thus 

 gained seems to be employed partly in dissociating some of the 

 molecules of the gas (or in inducing some other condition which 

 has the effect of rendering the neighl^ouring air in some degree 

 a conductor of electricity) and partly in originating an undulation 

 through the ether, which, as it takes its rise in phenomena so 

 disconnected as the impacts of the molecules of the air, must 

 furnish a large contingent of light waves of short wave-length. 

 The shortness in the case of these Becquerel rays appears to 

 approach without attaining the extreme shortness of ordinary 

 Rontgen rays. The reduction of the speed of the quick moving 

 molecules would cool the layer of air to which they belong ; 

 but this cooling would rapidly be compensated by radiation andl 

 conduction from the surrounding atmosphere ; under ordinary 

 circumstances the difference of temperature would scarcely 

 be perceptible, and the uranium would thus appear to per- 

 petually emit rays of energy with no apparent means of 

 restoration. 



The total energy of both the translational and internal 

 motions of the molecules locked up in quiescent air at ordinary 

 pressure and temperature is about 140,000 foot-pounds in each 

 cubic yard of air. Accordingly the quiet air within a room 

 12 feet high, 18 feet wide, and 22 feet long contains energy 

 enough to propel a one-horse engine for more than twelve 

 hours. The store drawn upon naturally by uranium and other 

 heavy atoms only awaits the touch of the magic wand of science 

 to enable the twentieth century to cast into the shade the 

 marvels of the nineteenth. 



Whilst placing before you the labours and achievements of 

 my comrades in science I seize this chance of telling you of 

 engrossing work of my own on the fractionation of yttria to 

 which for the last eighteen years I have given ceaseless attention. 

 In 1883, under the title of "Radiant Matter Spectroscopy," I 

 described a new series of spectra produced by passing the 

 phosphorescent glow of yttria, under molecular bombardment 

 in vacuo, through a train of prisms. The visible spectra in 

 time gave up their secrets, and 'were duly embalmed in the 

 Philosophical Transactions. At the Birmingham meeting of 

 the British Association in 1886 I brought the subject before the 

 Chemical Section, of which I had the honour to be President. 

 The results led to many speculations on the probable origin of 

 all the elementary bodies— speculations that for the moment I 

 must waive in favour of experimental facts. 



There still remained for spectroscopic examination El long 

 tempting stretch of unknown ultra-violet light, of which the 

 exploration gave me no rest. But I will not now enter into 

 details of the quest of unknown lines. Large quartz prisms, 

 lenses, and condensers, specially sensitised photographic films 

 capable of dealing with the necessary small amount of radiation 

 given by feebly phosphorescing substances,^ and above all 

 tireless patience in collating and interpreting results, have all 

 played their part. Although the research is incomplete, I am 

 able to announce that among the groups of rare earths giving 

 phosphorescent spectra in the visible region there are others 

 giving well defined groups of bands which can only be recorded 

 jjhotographically. I have detected and mapped no less than 

 six such groups extending to A. 3060. 



Without enlarging on difficulties, I will give a brief outline 

 of the investigation. Starting with a large quantity of a group 

 of the rare earths in a state of considerable purity, a particular 

 method of fractionation is applied, splitting the earths into a 

 series of fractions differing but slightly from each other. Each 

 of these fractions, phosphorescing in vacuo, is arranged in the 

 spectrograph, and a record of its spectrum photographed upon 

 a specially prepared sensitive film. 



In this way, with different groups of rare earths, the several 

 invisible bands were recorded— some moderately strong, others 

 exceedingly faint. Selecting a portion giving a definite set of 

 bands, new methods of fractionation were applied, constantly 

 photographing and measuring the spectrum of each fraction. 



i In this direction I am glad to acknowledge my indebtedness to Dr. 

 Schuman, of Leipzig, for valuable suggestions and detail of his own 

 apparatus, by means of which he has produced some unique- records of 

 metallic and gaseous spectra of lines of short wave-length. 



