PHYSICS, PROGRESS OF, IN 1894. 



655 



on the combined atomic weights of the atoms 

 constituting certain groups. 



Phosphorescence. Pictet (Paris Academy of 

 Science, Sept. 24) has found that phosphores- 

 cent bodies cease to exhibit this property when 

 exposed to very low temperatures. The phos- 

 phorescence is regained, however, when the bodies 

 are allowed to assume their former temperature, 

 though in darkness. He regards these facts as 

 demonstrating that the phenomenon is due to 

 molecular movement. H. Ebert (Wiedemann's 

 " Annalen," September) has investigated the con- 

 ditions of obtaining the best luminous effects by 

 phosphorescence excited by electric radiation, and 

 points out that the secondary circuit must be tuned 

 to the primary, and that the condensers must 

 have the least possible capacity. He has devised 

 a lamp made of a glass globe containing a piece 

 of phosphorescent material. Oscillations are 

 conducted to tinfoil armatures on the globe, and 

 produce vivid luminescence. The economy is 

 very striking, but alternating currents of very 

 high frequency are required, necessitating the 

 employment of a transformer near the lamp, or 

 perhaps in direct connection with it. 



Visibility. P. L. Gray (London Physical So- 

 ciety, April 13) concludes from experiments: 

 (1) That the minimum temperature of visibility 

 is the same for a bright, polished surface as for 

 one covered with lampblack, though the radia- 

 tion may differ ; (2) that the visible limit at the 

 red end'of the spectrum varies with the state of 

 preparation of the eye, exposure to bright light 

 diminishing sensitiveness and darkness increas- 

 ing it ; (3) that for the less sensitive condition 

 the minimum temperature of visibility for the 

 surface of a solid is about 470" C. ; (4) that at 

 night a surface at 410 C. is visible, and by rest- 

 ing the eyes in complete darkness one at 370 

 may be seen : (5) that observers' eyes differ some- 

 what in their minimum temperature of visi- 

 bility. 



Standards. Sharp and Turnbull (" Physical 

 Review," July-August), as the result of a care- 

 ful study of light standards, conclude that it 

 is futile to attempt to obtain concordant pho- 

 tometric results from freely burning candles, 

 though the English is more stable than the 

 German. The most satisfactory results are 

 given by standards that have chimneys to pro- 

 tect the flame. 



Electricity. Electrification. Lork Kelvin 

 and Magnus Maclean (London Royal Society, 

 May 31) find that air can be electrified positively 

 or negatively, but that it does not retain nega- 

 tive electrification so long as positive. For sta- 

 ble equilibrium it is necessary that the electric 

 density, if not uniform throughout, diminish 

 from the bounding surface inward. Hence a 

 portion of non-electrified air must be wholly 

 surrounded by electrified air. Prof. J. J. Thom- 

 son (" Nature," July 26) points out that these ex- 

 periments prove simply that a gas can be elec- 

 trified, and do not disprove the conclusion 

 reached by him in his " Recent Researches in 

 Electricity and Magnetism " that a molecule of 

 gas can not be electrified, and that the charge 

 of a gas is carried by separated atoms. J. J. 

 Thomson (" Philosophical Magazine," April) con- 

 firms the idea already advanced by Lenard that 

 the electrification developed by splashing drops 



is due, not to friction, but to a previous double 

 coating of electricity on the drop, the water sur- 

 face having one charge and the gas in contact 

 with it an equal and opposite one. The splash- 

 ing dislodges some of the external coating. 

 This double coat, which is probably due to a 

 tendency to chemical action, is possessed by the 

 most diverse liquids, as water, turpentine, mer- 

 cury, and molten metals. It may be on solids, 

 too, in which case frictional electrification may 

 be due largely to the rubbing off of the external 

 electric coating. 



Conduction. G. M. Minchin (London Phys- 

 ical Society, Nov. 24, 1893) has experimented on 

 the action of electro-magnetic radiation on films 

 containing metallic powders. Such a film in 

 circuit with battery and galvanometer ordinarily 

 acts as an insulator, but when the electrodes are 

 brought very near together on the surface and 

 one of them is touched with an electrified body a 

 current passes. The electrodes may then be 

 separated a little more and the process repeated 

 till any desired extent .of the film is rendered 

 conducting. If the circuit be broken at the 

 film, it becomes an insulator again, but if else- 

 where it remains a conductor. A spark has an 

 effect similar to that of an electrified body, and 

 Prof. Minchin ascribes both to electric oscilla- 

 tions in the wires. Prof. Lodge, however, sug- 

 gests that the phenomena are due to the increase 

 of the range of molecular attraction, caused by 

 electric polarization. The phenomena are akin 

 to those of Minchin's impulsion cells (" Annual 

 Cyclopaedia," 1890, p. 717, and 1891, p. 730) and 

 of the conductivity of powders (1891, p. 731). 

 Braun ("Zeitschrit't fur physikalische Chemie," 

 February) has examined compound gases at the 

 moment of their formation, and finds no con- 

 ductivity when a Leyden battery is used with 

 nitrogen dioxide and air : but when chlorine and 

 hydrogen were tested with a Grove battery at 

 the moment of explosion they were found to 

 possess conductivity. When heated to 1,000 to 

 1.200 C., ammonium chloride and cadmium 

 iodide conducted well, others only fairly or not at 

 all. H. S. Carhart (" Physical Review," March- 

 April) concludes that Sanford's experiments on 

 the effect of the surrounding medium on the 

 conductivity of a conductor (" Annual Cyclopae- 

 dia," 1893, p. 621) are affected by some systemat- 

 ic error. Experiments of his own with more 

 delicate methods than Prof. Sanford's show no 

 such results as his. Bruno Piesch (Vienna ; ' Be- 

 richte," May 25) finds that in acids and so- 

 lutions of salts increase of pressure produces de- 

 crease of resistance, but the decrease is less as 

 the pressure increases. Polarization in general 

 increases with the pressure. C. V. Burton (Lon- 

 don Physical Society, April 27) has proposed a 

 theory of the mechanism of electrical conduc- 

 tion that is set forth in the following theorems, 

 which he deduces from generally accepted the- 

 ories of dielectrics and of magnetism : " I. In a 

 region containing matter there may be (and 

 probably always are) some parts which are per- 

 fect insulators and some parts which are perfect 

 conductors, but there can be no parts whose 

 conductivity is finite unless every finitely con- 

 ductive portion is inclosed by a perfectly con- 

 ductive envelope. II. In metals and in other 

 non-electrolytes whose conductivity is finite the 



