58 



SCIENCE. 



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



of gases by kathode and X-rays. Assuming 

 that the same energy is required to produce 

 ionization by the Becquerel rays, they esti- 

 mate that a sq. cm. surface of a thick layer 

 of uranium oxide gives off energy at a rate 

 not less than 10~" calorie per second, which 

 is suflicient to raise the temperature of 1 

 gram of water 1° C. in 3,000 years. In the 

 case of radium, 100,000 times as active as 

 uranium oxide, the energy given off is not 

 less than 3,000 calories per year for a gram 

 of substance. This value for the energy is 

 40 or 50 times as large as the value men- 

 tioned above, obtained by the other method. 

 That the amount of energy concerned is 

 so small emphasizes the extreme delicacy 

 of the photographic and ionization tests. 



SECONDARY RADIATIONS. 



X-rays impinging on some substances in- 

 duce secondary radiations, and it was soon 

 found that the Becquerel rays also possess 

 this power, with the important difference 

 that the secondary radiations induced by 

 the Becquerel rays continue after the action 

 of the primary rays has ceased, which is 

 not the case with those induced by the X- 

 rays. These secondary radiations affect 

 photographic plates and ionize gases. The 

 radium preparations are the most active in 

 producing secondary radiations, and the ef- 

 fect is produced equally well in the case of 

 such different substances as platinum, zinc, 

 bismuth and even paper. Up to a certain 

 limit the intensity of the secondary rays 

 increases with the time of exposure to the 

 radium ; after the removal of the radium 

 the intensity gradually weakens, disappear- 

 ing altogether after some hours. The source 

 of the secondary rays was thought to be a 

 fine dust which escapes from the radium 

 and settles on neighboring bodies, but it 

 cannot be removed by washing, and the 

 radium rays have the power of imparting it 

 even after .passing through metal screens. 

 It seems too that this power of emitting 



radiations may even be imparted to gases. 

 A number of substances may be made 

 strongly radio-active by precipitation from 

 solutions containing small amounts of the 

 most active substances, and several times 

 these temporarily active substances have 

 been mistaken for compounds of new ele- 

 ments. It is not yet known to what extent 

 the secondary radiations are like the 

 primary, for their intensity is so small that 

 a comparison is difiicult. 



Thorium oxide gives off a remarkable 

 vapor or ' emanation ' which causes a 

 strong secondary radio-activity. This ema- 

 nation can pass through paper and even 

 very thin metal. It gradually diffuses itself 

 throughout the air and is carried about 

 by air currents. Air containing the ema- 

 nation retains its electrical conductivity for 

 as long as ten minutes after the thorium 

 oxide is removed, though ordinary ionized 

 air loses its conductivity in a few seconds. 

 The emanation is not removed from the air 

 by drawing it through wool or bubbling it 

 through water or sulphuric acid. Any sub- 

 stance charged with negative electricity 

 collects and concentrates the emanation, 

 becoming very radio-active after a few 

 hours in the presence of thorium oxide. 

 Sand-papering Or treatment with sulphuric 

 acid removes the emanation from a platinum 

 wire on which it has been concentrated. 

 On afterwards evaporating the acid a radio- 

 active residue may be obtained. The 

 thorium emanation much resembles a fine 

 radio-active dust. 



NATURE OF THE RADIATIONS AND SOURCE OF 

 THEIR ENERGY. 



We may say then that for one com- 

 ponent of these complex radiations a satis- 

 factory explanation is offered. This com- 

 ponent is of the same nature as the kathode 

 rays, and consists of a rapidly moving 

 stream of minute material particles each 

 having its charge of negative electricity. 



