72 MORRIS LOEB 



obtained, we cannot consider the magnitude of the momen- 

 tum as definitely established; especially since their speed 

 does not appear to be uniform. From experiments wherein 

 a particles are allowed to escape freely, and again restrained 

 by a lead cylinder surrounding radium, much of the appar- 

 ent heat of the latter body appears to be due to the impinging 

 of the a-rays upon the surrounding surfaces. 



/3-rays are similar to cathode rays; they are less absorb- 

 able than the a variety, and proceed at various speeds, many 

 approaching the velocity of light; they are stopped by solids 

 in proportion to their density. 



7-rays are similar to X-rays, of great penetrating power, 

 and they are thought by some to be secondary effects of a- 

 and /3-rays, just as the X-rays originate from the impact of 

 cathode rays on the glass wall of the Crookes tube. Besides, 

 we have a multitude of conflicting accounts of secondary 

 tertiary rays, resulting from these three varieties. 



The chief method of research is the study of ionization, 

 with the interposition of screens and magnetic fields, to 

 separate the different kinds of rays. On the other hand, the 

 varieties of rays emitted, their relative strength, and their 

 variations of intensity, are the characteristics upon which 

 the identification of the various so-called transformation- 

 products of radioactive material is based. I have, therefore, 

 copied from Professor Rutherford's book 1 tabulations of these 

 properties. 



With regard to these various transformations, we should 

 realize that the majority of the names are titles of hypothet- 

 ical substances whose existence within certain mixtures is 

 assumed upon the evidence of their momentary radioactiv- 

 ity. The only one really isolated is that emanation which has 

 all the properties of a gas, including that of condensibility 



1 Radioactivity, 1905. 



