732 



PHYSICS, PROGRESS OF, IN 1891. 



Copper 2-52 



Platinum 2'02 



Nickel 1-29 



Iron 0-71 



Iridium 0'4T 



The above results are for equal surfaces. The 

 volatilities for equal weights, obtained by di- 

 viding by the specific gravities, are as follow : 



Palladium 9 



Silver T'88 



Tin 7-76 



Lead 6 - 61 



Gold 6'18 



Cadmium 3'72 



The order of electric volatilities does not cor- 

 respond to that of melting points or of any other 

 known constants. By the method of electric 

 evaporation and condensation, Prof. Crookes ob- 

 tained films of gold and silver that could be re- 

 moved in the form of brilliant foil. 



Inductive Discharge through Oases. Prof. J. 

 J. Thomson (London Physical Society, May 9) 

 finds that the electric discharge in rarefied gas 

 is greatly simplified when there are no electrodes, 

 He passes a Leyden-jar discharge through a tube 

 of mercury wound spirally around a gas tube, 

 thus electrifying the latter by induction, and finds 

 that the "dark space" and the stria? are absent. 

 The conductivity of the gas tube increases as 

 the pressure diminishes up to a certain point 

 and then diminishes, proving that the high re- 

 sistance of a nearly perfect vacuum is not due 

 to the presence of electrodes. A fine wire in the 

 tube on the side opposite to the primary circuit 

 prevents luminous discharge. These experi- 

 ments lead Prof. Thomson to conclude ("Philo- 

 sophical Magazine," October) that the distinc- 

 tion between electrostatic and electromagnetic 

 induction is to be made only for convenience. 

 The same phenomena that would be considered 

 electrostatic in a tube with free ends would be 

 called electromagnetic in an endless tube. Nik- 

 ola Tcsla (" Electric Engineer," New York, July 

 1) has made similar experiments by inserting in 

 an ordinary incandescent lamp bulb a tube con- 

 taining a roll of copper foil, through which the 

 jar discharge is passed. He concludes that the 

 effect is purely electrostatic. By so directing 

 the discharge as to exalt the electrostatic effect, 

 he produced light in a bulb several feet distant 

 from the primary, and has shown it to be possi- 

 ble thus to cons'truct an electric lamp without 

 visible connections that will be luminous in what- 

 ever part of a room it may stand. 



The Electric Arc. According to Elihu Thomp- 

 son (" Electric World," Feb. 28), this has not 

 received the study that it merits. He thinks 

 that the carbon is actually vaporized by the cur- 

 rent in the arc (see also Electric Evaporation 

 above), a coherent deposit being obtainable from 

 it. In long arcs there is an outer zone of true 

 combustion. The carbons in an electric arc light 

 become plastic, as can be shown by taking an 

 impression. The real work of the arc, as shown 

 by difference of potential, is in vaporizing 

 the carbon in the " crater " of the positive pole. 

 Photographs by J. C. McMynn, described by 

 E. L. Nichols in " The Electrician " (June 5), 

 show that the humming of an alternating cur- 

 rent arc is similar to that of a singing flame, 

 the arc being rapidly extinguished and relight- 

 ed. They show also an oscillation of the arc 

 from right to left, due to the influence of the 

 earth's magnetism. In an earlier article ("Amer- 

 ican Journal of Science," January) Mr. Nichols 

 noted that when the terminals of the secondary 



coil of an alternate-current transformer are 

 armed with a ball and a point, when the distance 

 is such as to admit of a discharge between them, 

 a shunt galvanometer shows a considerable con- 

 tinuous current from ball to point. F. C. Cald- 

 well finds that the discharge when the ball is pos- 

 itive, leaves perpendicularly to the surface, and 

 when the point is positive it leaves the apex and 

 reaches the ball obliquely. 



High-pressure Phenomena. Recent experi- 

 ments have been made in Frankfort, Germany, 

 and in London, with alternating currents of the 

 enormous potential of 45,000 volts. When the 

 electrodes were placed several inches apart and 

 a thick sheet of glass was inserted between them, 

 the discharge appeared like a true flame, darting 

 toward the glass and licking it with interlacing 

 tongues of fire. As the pressure increased the 

 flame flowed around the edges, and the glass was 

 finally shattered. 



Point Discharge. A. P. Chattock ("Philo- 

 sophical Magazine," September) says that where 

 electrified steel needle-points are discharged in 

 air the smooth, curved apex of the point is stud 

 ded with chains of air molecules (known as 

 " Grotthtis chains ") standing tip like bristles. 

 External pressure varies the length and close- 

 ness of the chains, and affects the discharge. 



Electricity from Carbon Dioxide. Dr. Hauss- 

 knecht (Berlin " Berichte," May) notices inter- 

 esting electric phenomena accompanying the 

 manufacture of solid carbon dioxide on a large 

 scale. The liquid gas is allowed to escape into 

 sail cloth or canvas bags. In the dark these ap- 

 pear illuminated by a pale greenish-violet light, 

 and sparks from 10 to 20 centimetres long dart 

 from the cloth. The phenomena seem to be 

 analogous to those of hydro-electricity, and to bo 

 due to the friction of the gas and spray on the 

 pores of the cloth. 



Dielectrics. E. Bouty (Paris Academy of Sci- 

 ences, April 27) finds that mica behaves as an 

 invariable dielectric in a direction normal to the 

 planes of cleavage. Between and 800 C. the 

 dielectric constant does not alter by -^,. It 

 is possible that between 300 and 400 C. mica 

 may possess some specific conductivity, but it is 

 negligible for charges of brief duration. The 

 large variations of mica condensers with the du- 

 ration of charge he finds to be due to electroly- 

 sis of foreign bodies in the superficial layer. 



Photography by Electric Spark. Lord Ray- 

 leigh, in a lecture before the Royal Institution 

 (Feb. 6), exhibited spark photographs of bub- 

 bles of a gas rising within a liquid and breaking 

 the surface into spray ; of jet phenomena ; and 

 of the breaking of a soap film. The success of 

 these pictures depended on a delicate electro- 

 magnetic adjustment which determined the pro- 

 duction of the spark at the desired moment. 



Apparatus for Measurement. Max Wien ("An- 

 nalen der Physik und Ohemie," iv) uses a tele- 

 phone to measure electric currents, the diaphragm 

 being replaced by a thin metal plate, like that of 

 an aneroid barometer, which is connected by 

 .levers to a mirror. A reflected beam of light 

 serves as an index. C. V. Boys (London Phys- 

 ical Society. June 26) has devised a pocket elec- 

 trometer, using his quartz fibers for suspension. 

 The advantage of a small instrument is that it 

 can be made 10,000 times as sensitive as a large 



