x] RADIO-ACTIVE PROCESSES 349 
alteration in the chemical and physical properties of the matter. 
The transformation of the radio-elements is thus a transformation 
of a part per saltum, and not a progressive change of the whole. 
At any time after the process of transformation has been in 
progress there will thus remain a part of the matter which is 
unchanged, and, mixed with it, the products which have resulted 
from the transformation of the remainder. 
The question naturally arises whether the process of degrada- 
tion of matter is confined to the radio-elements or is a universal 
property of matter. It will be shown in chapter x1. that all 
matter, so far examined, exhibits the property of radio-activity to 
a slight degree. It still remains to be shown, however, that the 
observed radio-activity is not due to the presence in the matter of 
a slight trace of a radio-element. If ordinary matter is radio- 
active, it is certain that its activity is not greater than that of 
uranium, and consequently that its rate of transformation must 
be excessively slow. There is, however, another possibility to be 
considered. The changes occurring in the radio-elements would 
probably never have been detected if the change had not been 
accompanied by the expulsion of charged particles with great 
velocity. It does not seem unlikely that an atom may undergo 
disintegration without projecting a part of its system with great 
velocity. In fact, we have seen that, even in the radio-elements, 
one of the series of changes in both thorium and radium is unac- 
companied by ionizing rays. It may thus be possible that all 
matter is undergoing a slow process of transformation, which has 
so far only been detected in the radio-elements on account of the 
expulsion of charged particles during the change. This process of 
degradation of matter continuing for ages must reduce the con- 
stituents of the earth to the simpler and more stable forms of 
matter. 
The idea that helium is a transformation product of radium, 
suggests the probability that helium is one of the more elementary 
substances of which the heavier atoms are composed. Sir Norman 
Lockyer, in his interesting book on “Inorganic Evolution,” has 
pointed out that the spectrum of helium and of hydrogen pre- 
dominates in the hottest stars. In the cooler stars the more 
complex types of matter appear. Sir Norman Lockyer has based 
