'08 



PHYSICS IN 1899. 



Photometry. C. Henry (Comptes Rendus, 

 CXXVIII, p. 941) has devised a photographic 

 actinometer consisting of a series of semitrans- 

 lucent and semiphosphorescent screens, 

 screen is first exposed to the source of light and 

 then placed at the end of a tube, where its lu- 

 minosity is compared with that of ground glass 

 illuminated by a standard candle. The rate at 

 which phosphorescent zinc sulphide loses its lu- 

 minosity being known, it is easy to determine 

 its original luminosity. Cattell (American As- 

 sociation) has utilized in photometry his obser- 

 vation that the time required to decide winch 

 of two surfaces is the darker is a direct measure 

 of the difference in luminosity of the two sur- 



Bccqucrel Rays. Rutherford (Philosophical 

 Magazine, XLVII, p. 109) finds no evidence of 

 polarization or refraction in these rays, thus dif- 

 fering from the earlier conclusions of Becquerel, 

 their discoverer. He explains the electrical con- 

 ductivity produced by the radiation by supposing 

 that the rays in traversing a gas produce in it 

 oppositely charged particles, and that the num- 

 ber of these in a second depends on the intensity 

 of the radiation and on the pressure. The radia- 

 tion is complex, one constituent (a) being readily 

 absorbed, while another () is more penetrative. 

 The former produces the greater amount of leak- 

 age of electricity between two metal plates, while 

 the latter is photographically more effective. The 

 cause of the radiation remains unknown, but the 

 energy spent seems to be extremely small. J. J. 

 Thomson suggests that the rays are due to elec- 

 trical effects arising from regrouping of atomic 

 constituents. Becquerel (Comptes Rendus, 

 CXXVIII, p. 771) in later experiments fails to 

 confirm his original results, according to which 

 the rays were regularly polarized, reflected, and 

 refracted. Preparations made in May, 185)0, have 

 -lost very little of their radio-active power. They 

 seem, like a magnet, to contain a store of energy 

 expended in producing their present condition, and 

 the process of giving out this energy is very slow. 

 It has not been possible to alter its intensity by 

 physical means. Elster and Geitel (Wiedemann's 

 Annalen, LXIX, 1, p. 83) find that Becquerel rays 

 are uninfluenced by the impact of cathode rays 

 or by temperature. The authors believe the rays 

 to be Rontgen rays of small intensity, but they 

 are not magnetically deflected. Behrendsen (ibid., 

 p. 220) adduces still more evidence in favor of 

 the identity of the rays with Rontgen rays. The 

 polarization of Becquerel rays, which their dis- 

 coverer had relied upon as a distinction from 

 Rontgen rays, has been disproved by this investi- 

 gator. Crookes (Comptes Rendus, CXXVIII, p. 

 17G) says that the radiation from uranium and 

 similar metals may be explained by supposing 

 that they are able to separate the rapidly mov- 

 ing from the slowly moving air molecules, and to 

 appropriate some of the energy of the former. 



Focometer.Deve (Comptes 'Rendus, CXXVIII, 

 p. 1501) has devised an instrument to measure 

 the curvatures, focal lengths, and aberrations of 

 optical surfaces. The optical system producing 

 the image observed is so arranged that it can 

 oscillate about an axis near the image, and its 

 position is altered till the image, as observed 

 through an eyepiece, does not oscillate with it. 

 It must then be on the axis, whose distance from 

 the optical surface is next measured. The pre- 

 cision is much greater than that of the ordinary 

 methods. 



Rky Colors, Bock (Wiedemann's Annalen, 

 LXVIII, 4, p. 674) has investigated the "blue 

 steam jet " described by Helmholtz, and finds its 



color strictly identical with that of the blue sky. 

 Under illumination similar to that of the atmos- 

 phere the jet shows a whitish color, which, when 

 observed at right angles to the ray, can be made 

 to disappear by turning the analyzer. The red, 

 yellow, and green components are polarized, and 

 the blue and the violet rays are unpolarized. the 

 result being a phenomenon analogous to Tyndall's 

 " residue blue." The transmitted ray shows red- 

 dish-yellow coloration. The jet is invisible in 

 red light, and is distinct in blue. Probably the 

 diameter of the droplets is equal to the wave 

 length of the blue rays. 



Electricity. Discharge Phenomena. llasehek 

 and Mache (Wiedemann's Annalen, LXVIII. 4, 

 p. 740) estimate the increase of pressure within 

 an electric spark to be about 11 atmospheres, 

 increasing with the energy, with the pressure of 

 the gas. and with the sparking distance. Then- 

 is a similar increase when an arc is established, 

 and in both cases it appears due not to heat l>ut 

 to particles projected from the electrodes. Ki- -ke 

 (ibid., p. 729) has measured the work consumed 

 in a spark gap, which he finds to be as follow*: 



Gap 2 cm 1 .64 watts Heat 4.96 grrm.-cal. per sec. 



' 4cm 2.08 " " 6.06 



6cm 3.27 



8cm 1.79 



.v; 

 :, : 



The diminution of the work from 6 to 8 centi- 

 metres is due to the diminished quantity of the 

 discharge. The potential continues to rise. Niko- 

 laieve (Journal de Physique, VIII, p. 432 ) Minis 

 that when a layer of cotton wool, pressed between 

 two ebonite rings, is placed in the path <>f a di- 

 ruptive discharge the wool is traversed by a 

 canal about t> millimetres in diameter, whose *id. - 

 are formed by compressed wool. If the wool is 

 soaked in water or vaseline the passage is larger. 

 Walter (Wiedemann's Annalen, LXVIII. I. p. 

 770) adduces additional evidence to support hi- 

 view that every spark discharge is pre< -eded by 

 tentative brush discharges from both electrode- 

 Oberbeek (ibid., LXVII, 3, p. 592), in an inv,-ti- 

 gation of the difference between spark ami l.ru-li 

 discharges, finds that on reducing the pie--uie 

 the character of both changes. Assuming that 

 the discharge that is attached to both electrodes i- 

 the spark discharge, he shows how and where tin- 

 two varieties merge. Almy (German Physical 

 Society), in experiments on spark discharge in 

 solid 'and liquid dielectrics, finds (1) that the 

 discharge potential is reduced by about "> pel- 

 cent, when the spherical electrode is replaced l.y 

 a pointed one; (2) that with dissimilar elect md e- 

 the discharge potential does not depend on re- 

 versal of polarity: and that (3) considerably 

 lower values of the discharge potential are ob- 

 tained if, instead of raising the potential grad- 

 ually, oscillations are allowed to take pla< by 

 connecting an air-spark gap in parallel with the 

 apparatus and increasing the gap until the di- 

 electric is pierced. Berthelot (Annales de Chunk 

 et Physique, XVI, p. 5) has subjected many or- 

 ganic "compounds to the action of the bru-h <li 

 charge (efflitve electrique). The bodies were placed 

 in a narrow space between armatures and sub- 

 jected to a noiseless and invisible discharge horn 

 a rapidly interrupted induction coil of 12-milli- 

 metre spark length for twenty-four hour-. In 

 general there was observed first a decomposition! 

 in which hydrogen and the simplest binary .mil- 

 pounds were separated, and then a condensation 

 or polymerization forming highly complex -lib- 

 stances. There is thus an analogy with the a< -t i"ii 

 to which food is subjected in the animal body. 



Phenomena in Rarefied Gases*. J. J. Thomson 

 (Cambridge Philosophical Society, June 7) de- 



