XI] AND OF ORDINARY MATERIALS Bye 
in general, which is only readily detectable when large quantities 
of matter are present. The experimental evidence is not yet 
sufficient to answer this question, but undoubted proof has been 
obtained that many of the metals show a very feeble radio-activity. 
Whether this radio-activity is due to the presence of a slight trace 
of the radio-elements or is an actual property of the metals them- 
selves still remains in doubt. This point will be discussed in 
more detail later in section 220. 
Schuster? has pointed out that every physical property hitherto 
discovered for one element has been found to be shared by all 
the others in varying degrees. For example, the property of 
magnetism 1s most strongly marked in iron, nickel, and cobalt, but 
all other substances are found to be either feebly magnetic or 
diamagnetic. It might thus be expected on general principles 
that all matter should exhibit the property of radio-activity in 
varying degrees. On the view developed in chapter x. the 
presence of this property is an indication that the matter is 
undergoing change accompanied by the expulsion of charged 
particles. It does not, however, by any means follow that because 
the atom of one element in the course of time becomes unstable 
and breaks up, that, therefore, the atoms of all the other elements 
pass through similar phases of instability. 
It has already been mentioned (section 8), that Mme Curie 
made a very extensive examination of most of the elements and 
their compounds for radio-activity. The electric method was 
used, and any substance possessing an activity of 1/100 of that of 
uranium would certainly have been detected. With the exception 
of the known radio-elements and the minerals containing uranium 
and thorium, no other substances were found to be radio-active 
even to that degree. 
Certain substances like phosphorus? possess the property of 
ionizing a gas under special conditions. The air which is drawn 
over the phosphorus is conducting, but it has not yet been settled 
whether this conductivity is due merely to ions formed at the 
surface of the phosphorus or to ions produced by the phosphorus 
nuclei or emanations, as they have been termed, which are carried 
1 British Assoc. 1903. 
* J. J. Thomson, Conduction of Electricity through Gases, p. 324, 1903. 
249 
