x] RADIO-ACTIVE PROCESSES 323 
this production are different from the laws of ordinary chemical 
reactions. It has not been found possible in any way to alter 
either the rate at which the matter is produced or its rate of 
change when produced. Temperature, which is such an important 
factor in altering the rate of chemical reactions, is, in these cases, 
entirely without influence. In addition, no ordinary chemical 
change is known which is accompanied by the expulsion of charged 
atoms with great velocity. It has been suggested by Armstrong 
and Lowry! that radio-activity may be an exaggerated form of 
fluorescence or phosphorescence with a very slow rate of decay. 
But no form of phosphorescence has yet been shown to be accom- 
panied by radiations of the character of those emitted by the 
radio-elements. Whatever hypothesis is put forward to explain 
radio-activity must account not only for the production of a series 
of active products, which differ in chemical and physical properties 
from each other and from the parent element, but also for the 
emission of rays of a special character. Besides this, it is necessary 
to account for the large amount of energy continuously radiated 
from the radio-elements. 
The radio-elements, besides their high atomic weights, do not 
possess In common any special chemical characteristics which differ- 
entiate them from the elements, which do not possess the property 
of radio-activity to an appreciable degree. Of all the known ele- 
ments, uranium, thorium, and radium possess the heaviest atomic 
weights, viz.: radium 225, thorium 232°5, and uranium 240. 
If a high atomic weight is taken as evidence of a complicated 
structure of the atom, it might be expected that disintegration 
might occur more readily in heavy than in light atoms. At the 
same time, there is no reason to suppose that the elements of the 
highest atomic weight should be the most radio-active; in fact, 
radium is far more active than uranium, although its atomic weight 
is less. This is seen to be the case also in the radio-active pro- 
ducts; for example, the radium emanation is enormously more 
active weight for weight than the radium itself, and there is 
every reason to believe that the emanation has an atom lighter 
than that of radium. 
1 Proc. Roy. Soc. 19038. 
