PHYSICS, PROGRESS OP, IN 1900. 



571 



lavized; in crystals it generally presents only netic deflection. The difference between Rontgen 



traces of polarization, except in topaz, in which and Becquerel rays may thus be, after all, quanti- 



it is completely polarized. Fluorescence of crys- tative rather than qualitative. Absorption and 



tals depends usually upon chemical impurities, magnetic deflection appear to be connected in 



probably distributed in accordance with struc- some way. Curves obtained in photographic 



ture. The color of a crystal has no influence experiments show that the speed of propagation 



upon fluorescence, except to affect the color, of radium radiation is of the order of the speed 



A magnetic field has no effect upon the color of of rotation communicated by a magnetic field of 



the fluorescent light, and causes nothing analo- about 4,000 units. This is probably very near the 



gous to the Zeeman effect. 



Becquerel Rays: Nature and General Properties. 



velocity found for cathode rays. Curie (Comptes 

 Rendus, Jan. 8) shows that the behavior of 



-M. and Mme. Curie (Comptes Rendus, Nov. 6, Becquerel rays in a magnetic field varies according 



1899) find that the rays from strongly radio-active to the mode of preparation of the radio-active 



ibstances are able to communicate radio-activity substance. Those that have most penetrative 



> substances that are ordinarily inactive, and that power are most easily deflected. M. and Mme. 



lis induced radio-activity persists for a consider- Curie (Comptes Rendus, March 5) find that the 



ible time. The effects seem not to be due to traces magnetically deflected rays of radium compounds 



radio-active matter carried on to the exposed resemble cathode rays rather than Rontgen rays, 



plate, but an induced radio-activity appears to not only in magnetic deflection, but also in con- 



dst, which is a sort of secondary radiation due veying a negative electric charge. Even after 



i the rays of Becquerel. Becquerel (Comptes traversing a thick- walled leaden box covered 



sndus, June 11) notes that two different kinds with a piece of aluminum foil, and also an ebon- 



the rays bearing his name have up to the ite layer 0.3 millimetre thick, the rays conveyed 



resent time been discovered. One resembles 

 ithode rays and is deflected both by magnetic 



a current of the order of 10~ u ampere through 

 the galvanometer connected with the inclosed 



id electric forces. The other is deflected by electrode. Strutt (Proceedings of the Royal So- 

 leither, and is absorbed in various degrees by ciety, 66, March 3) attempts to compare the mag- 



letals and other opaque substances. Radium 

 lits both, polonium only the second, and actini- 



netic deflections of Rontgen, Becquerel, and cath- 

 ode rays. He reports that with a field of 3,270 



m only the first. It seems probable that uranium C. G. S. units no perceptible deflection of Rontgen 



las a definite though feeble radiation of its own. 

 'illard (Comptes Rendus, April 30) has shown 

 heterogeneous nature of the rays given out by 



rays is obtained, and that to produce a curva- 

 ture of radius 1 centimetre in the rays would 

 require a field of at least 60,000,000 units. Thom- 



idium preparations, by allowing such rays to son has found that to produce the same curva- 



pass through a slit in a lead plate, and to im- 

 pinge at nearly grazing incidence upon two sen- 

 sitive plates laid one over the other and wrapped 

 in black paper. The rays are exposed to a strong 

 magnetic field. On development the upper plate 

 shows two impressions one deviated and spread 

 out, the other feebler, but absolutely rectilinear 

 and sharp. On the lower plate only one impres- 

 sion is visible that due to the nondeflected rays. 

 The author calls the two classes of rays simply 

 X rays and cathode rays. Mme. Curie (Comptes 

 Rendus, Jan. 8) finds that the rays are more 

 easily absorbed when they have already pene- 

 trated an absorbing layer than when they have 

 not, which is due to the absorption of the less 



ture in cathode rays a field of only 315 units 

 is required. Becquerel rays stand between Ront- 

 gen and cathode rays in this respect, a radius 

 of 1 centimetre requiring 5,000 units. Dorn 

 (Physikalische Zeitschrift, May 5), in experiments 

 on the behavior of radium rays in an electric 

 field, has shown that the rays always deviate 

 toward the positive plate of a condenser, like 

 cathode rays. The observed displacement agrees 

 fairly well with that calculated on the supposi- 

 tion that the rays consist of electrically charged 

 masses moving w T ith great velocity. 



Induced Radio-activity. Rutherford (Philo- 

 sophical Magazine, January) finds that thorium, 

 besides ordinary Becquerel rays, continuously 



penetrative rays in the first layers. Behrendsen emits radio-active particles of some kind, which 



(Annalen der Physik, June) finds that when a 

 radium preparation is cooled to the temperature 

 of liquid air its radio-activity is reduced by more 

 than one half. On heating again to the normal 



retain their powers for some minutes. This 

 emanation ionizes gas in its neighborhood, and 

 can pass through thin layers of metals and con- 

 siderable thicknesses of paper. It is also unaf- 



temperature a slight increase was observed. Els- fected by bubbling through water or sulphuric 



ter and Geitel (Wiedemann's Annalen, November, 

 1899) find that the rays of radium convert a spark 

 or brush electric discharge into a violet glow, 



acid. It can not be vapor of thorium, for the 

 emanation from thorium oxide was not sufficient 

 to appreciably alter the pressure of the gas in an 



acting sometimes at a distance exceeding one exhausted tube, and the spectrum of the gas was 



unchanged. The same writer (ibid., February) 

 finds that thorium compounds produce radio- 



letre. 

 Becquerel Rays; Magnetic and Electrostatic 



ejection. Becquerel (Comptes Rendus, Dec. 11, activity in all solid substances in their neighbor- 



~99) finds that the rays known by his name hood if the bodies are uncharged. With charged 



i' hen they pass along the lines of force of a conductors the radio-activity is produced on the 



powerful magnet are contracted to a focus. When negatively charged bodies. In strong electric 



the rays pass across the lines, after proceeding fields the radio-activity can be concentrated on 



Upward from the source, they curve round like the surface of thin wires. The radiation excited 



projectiles and impinge upon* the plate along a is homogeneous and more penetrating than the 



curve extending from one pole to the other and radiations from thorium or uranium. It is con- 



bending out of the way of the radiant substance fined to the surface of the substance, and is inde- 



in the center. These properties are analogous to pendent of whether the substance is a conductor 



ravs. The same writer (ibid., or not, and of the nature of its surface. The in- 



tensity of the induced radiation falls off in geo- 





those of cathode rays. 



Dec. 26, 1899) finds 'that the magnetic deflection 



of the rays is a property varying from one radi- metrical progression with the time, decreasing to 

 ant preparation to another. Polonium prepara- half value in about eleven hours. With time of 

 tions show no traces of it in the most powerful exposure the induced radio-activity at first in- 

 field, whereas radium shows very distinct mag- creases nearly proportionally, but tends to a 



