34 Professor Ernest Rutherford [Jan. 31, 



electrical methods. Unfortunately, it is very difficult to study the 

 rayless products with care, as in practically every case they are 

 succeeded by a ray product of comparatively rapid transformation. 

 The rayless products are of great interest as indicating the possibility 

 of transformations which can occur without any detectable radiation. 



In the course of the analysis of radio-active changes, special 

 methods have been developed for the separation of the various pro- 

 ducts from each other. It is only in a few cases, however, that we 

 can hope to obtain a sufficient quantity of the substance to examine 

 by means of the balance. It should be possible to obtain workable 

 quantities of actinium, radium D (radio-lead), and radium CI 

 (polonium), but the isolation of these substances in any quantity has 

 not yet been eifected. Sir William Ramsay and Mr. Cameron have 

 made a number of important investigations of the properties and 

 volume of the radium emanation, freed so far as possible from any 

 traces of known gases. The remarkable initial contraction of the 

 volume due to the emanation shows that there is still much to be 

 done to obtain a clear understanding of the behaviour of this 

 intensely radio-active gas when obtained in a pure state. 



Simultaneously with the work on the analysis of radio-active 

 changes, a large number of investigations have been made on the 

 laws of absorption by matter of the three primary types of radiation 

 from active matter, viz. the a, y8, and y rays, and the secondary 

 radiations to which they give rise. It has generally been accepted 

 for some years that the y rays are a type of penetrating X-rays. The 

 latter are supposed to consist of electro-magnetic pulses in the ether, 

 set up by the impact or escape of electrons from matter, and akin in 

 many respects to very short waves of ultra-violet light. Recently, 

 however, Bragg has challenged this view, and has suggested that the 

 y rays (and probably also the X-rays) are mainly corpuscular in 

 character, and consist of uncharged particles, or " neutral pairs," as 

 he terms them, projected at a high velocity. Such a view serves to 

 explain most of the experimental observations equally well as the 

 pulse theory ; Bragg has recently brought forward additional evidence, 

 based on the direction of the secondary radiation from the y rays, 

 which he considers to be inexplicable by the pulse theory. AVe must 

 await further data before this important question can be settled 

 definitely, but the theory of Bragg, which carries many important 

 consequences in its train, certainly deserves very careful examination. 



From the radio-active point of view, the a rays are by far the 

 most important type of radiation emitted by active matter, although 

 their power of penetration is insignificant compared with the /i or y 

 rays. They consist of veritable atoms of matter projected at a speed, 

 on an average, of 6000 miles per second. It is the great energy of 

 motion of these swiftly expelled masses that gives rise to the heating 

 effect of radium. In addition, they are responsible for the greater 

 part of the ionisation observed near an uncovered radio-active sub- 



