S72 



PHYSICS, PROGRESS OF, IN 1900. 



maximum value. Mr. Rutherford suggests three 

 possible explanations: (1) A kind of phosphor- 

 escence excited by the radiation; (2) deposition 

 of the positively charged gaseous ions produced 

 in the gas; (3) deposition of particles of a radio- 

 iictive material emitted by thorium compounds. 

 The first appears insufficient, and between the 

 others the evidence is not conclusive, though the 

 last seems somewhat more tenable. (See also 

 Rontgen Rays, under ELECTRICITY.) 



Magneto-optics. Larmor (Cambridge Philo- 

 sophical Society Proceedings, 10, 1899) points out 

 that the Faraday effect may be deduced from the 

 Xccinan effect by general reasoning, without in- 

 troducing any ' special dynamical hypothesis. 

 Voigt (Annalen der Physik, February) also has 

 extended his magneto-optic theory, which in- 

 volves very complicated formulae. Reese (Elec- 

 trical World and Engineer, Aug. 18) has at- 

 tempted to determine whether the separation of 

 the spectral lines in the Zeeman effect is strictly 

 proportional to the strength of the field. Photo- 

 graphs show that it is very nearly so with sharp 

 triplets, less so with sextuplets, and still less 

 with diffuse triplets, the separation increasing 

 less rapidly with the field when the latter be- 

 comes very intense. 



Spectroscopy. Beckmann (Zeitschrift fur 

 physikalische Chemie, Sept. 7) obtains colored 

 flames for spectroscopic work by first forcing air 

 through the pores of a porcelain tube impreg- 

 nated with the salt to be examined and then 

 passing it through a Bunsen flame. With aqueous 

 solutions the necessary pressure is from l.G to 2 

 atmospheres, but with alcohol, acetone, or acetic 

 acid it is only about half as great. Cantor (An- 

 nalen der Physik, March) finds by experiment 

 that there is no perceptible absorption of the 

 light from an electric arc by glowing gas in a 

 vacuum tube. As by Kirchhoff's law the ratio 

 of the emission to the absorption equals the emis- 

 sion of a black body at the same temperature, 

 the glowing gases should have an extremely high 

 temperature. But since they have not, Kirchhoff's 

 law does not hold for electrically glowing gases. 



Microscopy. Deschamps ( Comptes Rendus, 

 April 30) has given the name telemicroscope 

 to an instrument which enlarges by about 12 

 diameters objects placed at a distance of 25 

 centimetres. It thus enables the observer to 

 study insects without disturbing them from 

 their normal surroundings. The objective con- 

 sists of two Dollond achromatic lenses separated 

 by a distance less than the focal length of the 

 most converging lens. The eyepiece is a lens 

 of short focus, chosen so as to give the maxi- 

 mum enlargement and field. The depth of focus 

 of the instrument is very great, and it gives 

 effects of relief similar to stereoscopic effects. 



I'lititotropi/. This name is given by Marckwald 

 (Physikalische Zeitschrift, 1, 1899) to change 

 produced by light when the removal of the active 

 rays cause the substance acted upon to return to 

 its original condition. A new example is afforded 

 by chino-chinolin, whose yellow crystals become 

 given in the light, but turn yellow again in the 

 dark. Direct sunlight changes them in a few 

 seconds; diffuse daylight in several minutes. The 

 rate of return depends on temperature. 



Electricity. l-:ii-ctri/irnliiiii. K\>ri and Hoff- 

 mann (Annalen der Physik, August) find that 

 ice acquires a positive charge by friction with a 

 metal and imparts a negative charge. The colder 

 and drier it is, the greater this action. This 

 may account for the electric effects of polar snow- 

 -turnis, including atmospheric luminescence and 

 even the aurora; and the phenomenon should be 



allowed for in all experiments with liquid air. 

 which usually carries minute particles of very 

 cold ice suspended in its vapor. Campetti (Acad- 

 emy of the Lincei, July 15) finds that when a 

 crystal of a salt is dropped into water, or into 

 its own nonsaturated solution, the surrounding 

 liquid becomes negatively charged, and the crys- 

 tal positively charged. This he believes to be due 

 to the fact that the negative ions fly off more 

 rapidly than the positive. The resulting charges 

 accelerate the outflow of the positive ions and 

 retard the others, so that there is a tendency to 

 equalization, but on the whole the liquid is nega- 

 tive to the crystal. The results of an invest Ra- 

 tion of the masses of ions undertaken by J. J. 

 Thomson (Philosophical Magazine, December. 

 1899) suggest that the ionization of a gas con- 

 sists in the detachment from the atom of a nega- 

 tive ion the same for all gases whose mass is 

 only about y ff of the mass of an atom of hydro 

 gen. This negative ion may be the fundamental 

 quantity in terms of which all electrical procc-se> 

 can be expressed. In an electrical theory embody 

 ing this conception the atom would be regarded 

 as containing a large number of similar bodies. 

 which Thomson calls " corpuscles." The mass of 

 a corpuscle is about 3 X 10- 26 grammes. In the 

 normal atom this assemblage of corpuscles is 

 electrically neutral. The electrification of a gas 

 may be the detachment of a corpuscle from some 

 of the atoms. The remainder of the atom forms 

 the positive ion, and is of course of much greater 

 mass. An atom might be split up indefinitely 

 but for the fact that each detachment of a cor- 

 puscle increases the free positive charge of the 

 remainder, and so increases the force of cohe>i<in. 



Dissipation of Charge. Pochettino and Sella 

 (Academy of the Lincei, July 1) conclude that 

 the evaporation of a charged liquid does not dis- 

 sipate the charge, thus confirming the conclusions 

 of Schwalbe and controverting those of Pellat. 

 When the surrounding air is kept dry the addi- 

 tion of water to a zinc plate reduces the rate of 

 dissipation instead of adding to it. Elster and 

 Geitel (Annalen der Physik, July) find that the 

 dissipation of an electric charge is due to ions in 

 the atmosphere, and that it is reduced by fog. 

 smoke, and dust. It takes place equally for posi- 

 tive and negative charges, except at great alti- 

 tudes, where the rate may be two to ten times 

 greater for the negative than for the positive. 

 The authors believe that the higher strata of the 

 air are ionized by the sun's rays, the ultra-viol 

 rays, which produce this effect, being reduced 

 the lower strata by absorption. 



Continuous Dixcluiriic. Toepler (Annalen 

 Physik, July) finds that there may be four \ 

 ferent forms of continuous discharge in ai 

 glow, brush, brush-arc, and flame-arc. Each in; 

 be either positive or negative. Usually an i 

 crease in the strength of the current brings about 

 a change from glow to brush, and then to brush- 

 are. The author believes that so-called "globe 

 lightning" or "ball lightning" is due to a eon 

 tinuous discharge analogous to the brush-arc, the 

 way for which lias previously been cleared by a 

 discontinuous discharge of ordinary or "forked 

 lightning. The luminous globe is simply a por- 

 tion of the continuous track where there i- a 

 particularly steep gradient of potential, and it* 

 motion may be due to electrostatic forces or to 

 the motion of the air. carrying the whole track 

 with it. Generally the phenomenon ends with 

 another discontinuous or spark discharge, ami 

 the resulting detonation is ascribed to the " hurl- 

 ing" of the ball. The current intensity may bi- 

 as high as 10.000 amperes, but usually does not 



