56 



SCIENCE. 



[N. S. Vol. XIV. No. 341. 



tliinnest metal foil. Uranium radiations, 

 and therefore Crookes' UrX radiations, are 

 much more penetrating, passing through 

 metals, glass and in fact all substances, 

 but with considerable loss of intensity. 

 Thorium compounds emit radiations of 

 at least two very different penetrating pow- 

 ers, one part only feebly penetrating, an- 

 other as penetrating as the TJrX radia- 

 tions. Eadium and actinium also emit 

 both penetrating and non-penetrating rays, 

 some of the radium rays being the most 

 penetrating of all. Screens of sheet metal 

 act as sieves for the rays, soon cutting 

 off the less penetrating rays and allowing 

 the more penetrating kind to go through 

 with but little diminution in intensity. 

 One sheet of tin, 0.0025 mm. thick, trans- 

 mitted 44% of the radiations from one 

 radium specimen, two sheets of the same 

 thickness transmitted 31%, and 15 sheets 

 15%. A sheet of glass 0.16 mm. thick 

 transmitted 26%, and ten plates 16%. 

 Aluminum 0.16 mm. thick transmitted 

 28%, six sheets 16%. On account of the 

 non-homogeneity of the radiations it has 

 been very difficult to determine the law of 

 absorption, but it appears that for the rays 

 of the most penetrating type, at least, the 

 absorption is proportional only to the den- 

 sity and thickness of the absorbing screen, 

 the kind of material, whether platinum, 

 paper, glass, air or other substance, making 

 but little difference. Because some of the 

 less penetrating rays are absorbed by the 

 salt itself, there is a larger proportion of 

 the very penetrating rays in the radiations 

 from a thick layer of the salt than in those 

 from a thin layer. 



DEFLECTIOISr IN A MAGNETIC FIELD. 



Several experimenters discovered about 

 the same time that some of the Becquerel 

 rays are affected by a magnetic field. This 

 brought out strongly their resemblance to 

 the kathode rays, and further experiment- 



ing proved that if a beam of radium radia- 

 tions is made to pass through a magnetic 

 field which is perpendicular to the direc- 

 tion of the beam, then the beam is deflected 

 just as a beam of kathode rays would be, 

 that is, just as a stream of negatively 

 charged atomic projectiles would be deflect- 

 ed. This fact furnished the basis for the 

 present accepted hypothesis, namely, that 

 the deviable rays consist of a stream of 

 rapidly moving particles, charged with 

 negative electricity. The deflection in a 

 magnetic field gives further proof of the 

 non-homogeneity of the radiations. The 

 experiment is as follows : A vertical beam 

 is obtained by placing the radio-active salt 

 at the bottom of a narrow hole in a block 

 of lead, which is then placed on a hori- 

 zontal photographic dry-plate or a fluores- 

 cent screen, in the horizontal field of a large 

 electro-magnet. "When the magnet is en- 

 ergized the vertical beam of rays is deflect- 

 ed in a direction always perpendicular to 

 its direction of propagation, and also per- 

 pendicular to the magnetic lines of force, 

 so that it is finally bent over until it falls 

 upon the plate or screen. The impression 

 produced is not a spot, but a band or mag- 

 netic spectrum, which could not be the case 

 if the beam were composed of homogeneous 

 radiations. According to the electrified- 

 projectile hypothesis this can be explained 

 by saying that the particles do not all have 

 the same velocity, in which case those hav- 

 ing the highest velocity would be the least 

 deflected. This view has support in the 

 fact that of the deviable rays, the least de- 

 viable are the most penetrating, as we 

 should expect from the higher velocity of 

 the particles. Becquerel found that for the 

 rays from a sample of radium the product 

 of the strength of the field into the radius 

 of curvature of the path varied from 350 

 to 3,000. 



There are, however, certain raj^s that are 

 not deflectable in a magnetic field, and 



