550 



PHYSICS, PROGRESS OF, IN 1902. 



a scarcely appreciable fluorescence, from the 

 greatly inferior rubies of Burma, which are very 

 fluorescent and gleam brilliant red. W. Voigt 

 (Archives Neerlandaises, 6, 1901) states that 

 while a complete theory of fluorescence and phos- 

 phorescence on the electron hypothesis is at pres- 

 ent impossible, owing to our scanty knowledge 

 of the internal constitution of molecules, we may 

 be able, by the aid of experimental results, to 

 obtain certain generali/ations. The author con- 

 siders the two phenomena to be caused by irregu- 

 lar vibratioos due indirectly to the incident light, 

 although not directly excited by it. Free nega- 

 tive electrons would be more susceptible than 

 the more massive portions of the molecules, and 

 it might be assumed, therefore, that the latter 

 are indirectly set in vibration by the former. It 

 is more probable, however, that the molecules of 

 a fluorescent substance can exist in two or more 

 distinct states, to which correspond distinct in- 

 trinsic periods of the electrons. In such a medi- 

 um, if there is sensibly less damping in the state 

 corresponding to the longer period of the elec- 

 trons, there will be sensible fluorescence, and in 

 the other sensible absorption. 



Colors of Films. R. W. Wood (Philosophical 

 ^lairazine, April) has made "metallic deposits on 

 glass, which the microscope shows to be made up 

 of particles smaller than the wave-lengths of 

 light, and which by transmitted light exhibit col- 

 ors quite as brilliant as those produced by aniline 

 dyes." The deposits are obtained by heating 

 fragments of the alkali metals in exhausted and 

 sealed glass bulbs. Some of the more transparent 

 films, examined microscopically, were seen to be 

 composed of barely visible particles, lying close 

 together. When air is admitted the color van- 

 ishes, but hydrogen has no effect. The diameter 

 of the particles varies from 0.0003 to 0.0002 milli- 

 meters. The investigator finds that while coarse 

 particles diffract or scatter the light, closely 

 packed minute particles reflect those wave-lengths 

 absent in the transmitted light, and minute par- 

 ticles far apart diffuse light of the same wave- 

 lengths as those which are, to some extent, absent 

 in the transmitted light. Heating and cooling 

 both shift the absorption region of the spectrum 

 in the direction of greater wave-lengths, but the 

 effect due to cooling is temporary. A pale-green 

 film becomes deep violet when cooled. In some 

 films change and return of color may be effected 

 by blowing, and then rubbing with the finger. 

 The films are practically non-conductors of elec- 

 tricity, and the author suspects the phenomena 

 to be due to a kind of electrical resonance. 



Reversible Actions. E. Goldstein (German 

 Physical Society, 3, 14, 1901) finds that colors 

 produced in glass by cathode rays are destroyed 

 by ultra-violet light or by sunlight, and that 

 changes produced bj ultra-violet light are re- 

 versed by waves of greater length. The colors 

 can also be destroyed by heating. Bromide of 

 silver blackened by cathode rays and afterward 

 exposed to sunlight in a closed tube for three- 

 quarters of an hour has its original color entirely 

 restored, except at the illuminated surface. In 

 diffused light this result is reached in about two 

 days, and with chlorid of silver months are re- 

 quired, lodid of silver blackens and liberates 

 iodin under cathode rays; but a few days' day- 

 light regenerates the yellow iodid and a few sec- 

 onds under the positive light of the discharge 

 will do the same if the blackened iodid be 

 warmed. 



Action on Metallic Surfaces. H. Buisson 

 (Journal de Physique, October, 1901, and Eclair 

 Electrique, Oct. 5, 1901) finds that when a metallic 



surface is illuminated by sunlight, the rate of loss 

 of negative electric charge steadily falls, until at 

 length it ceases. In the dark this change to an 

 insensitive condition does not occur, and the sen- 

 sitiveness of a fresh surface is increased by keep- 

 ing it in darkness. Under the same radiations, a 

 metal becomes more negative by from 0.10 to 

 0.14 volt, with the exceptions of nickel, which is 

 very slightly affected, and platinum, which is 

 oppositely affected. This modification disap- 

 pears gradually and completely in the dark, and 

 under the continued action of light it reaches a 

 limit after a few minutes. The change produced 

 by light is superficial, the layer affected not being 

 even thick enough to alter the polarization of a 

 reflected beam. Probably the change is deter- 

 mined by an equilibrium between the surface of 

 the metal and the adhering film of gas. 



Radio-activity. (See also Rontyen Rays under 

 ELECTRICITY.) W. Crookes (London Royal So- 

 ciety, March 21) asserts that electrons from 

 radio-active bodies are impeded by the molecules 

 of the surrounding medium like material parti- 

 cles, while ether waves are not thus affected, ex- 

 cept by absorption. Actinium and radium are 

 found by the author to give electrons similar to 

 a fog or mist. When not kept in by a thick 

 metal screen, this diffuses away in the free air 

 like odoriferous particles. Polonium, however, 

 behaves somewhat differently. Radium emana- 

 tion may be removed by a current of air. It 

 will pass through aluminum and a considerable 

 length of air, and then affect a sensitive film, but 

 polonium differs h.ere also in its action. Cor- 

 puscles from polonium may be heavy positive 

 ions, and the author is now making experiments 

 to test this inference. From a negative silver 

 pole in a tube of very high vacuum with a per- 

 forated sheet of mica in front of it electrons 

 shot in all directions, and, passing through the 

 hole, formed a bright phosphorescent patch on 

 the opposite side of the tube. After some hours 

 silver had been deposited only on the mica screen 

 and near the pole, while the glowing end of the 

 tube was free of silver deposit. Electrons thus 

 seem to have been shot off from the negative 

 pole, causing the glass on which they impinged 

 to phosphoresce, while at the same time the 

 heavy positive ions of silver, freed from their 

 negative electrons, also flew off, and were depos- 

 ited near the pole. These metallic ions when 

 deposited on a metal plate in all cases sho\\t-<l 

 positive electrification, thus lending support to 

 the view of Strutt that the non-deflectable Becque- 

 rel rays are streams of heavy positive ions. M. 

 and Mme. Curie (Comptes Rendus, Jan. 13) look 

 upon radio-activity as an atomic property of bod- 

 ies. According to their latest hypothesis, each 

 atom of a radio-active substance acts as a constant 

 source of energy. The radiant activity seems to 

 be rigorously the same whenever the radio-active 

 body is brought back to the same chemical and 

 physical state, and appears not to vary with time. 

 Suppositions as to the origin of the energy of 

 radio-activity group themselves round two hy- 

 potheses: (1) Each radio-active atom possesses 

 in the condition of potential energy the energy 

 which it sets in action; (2) the radio-active atom 

 is a mechanism which draws at each instant 

 from outside of itself the energy which it sends 

 out. On the first hypothesis the potential energy 

 should in the long run become exhausted, but 

 the experience of years has hitherto not indi- 

 cated any variation. On the second hypothesis 

 radio-active bodies are transformers of energy. 

 This might be taken, contrary to the principle of 

 Carnot, from the heat of the surrounding medi- 



