XE] AND OF ORDINARY MATERIALS 381 
by removing the surface of the metal or by chemical treat- 
ment. 
It must be borne in mind that the activity observed in ordinary 
matter is excessively minute. The lowest rate of production of 
ions yet observed is 10 per cubic centimetre per second in a 
brass vessel. Suppose a spherical brass vessel is taken of capacity 
1 litre. The area of the interior surface would be about 480 sq. 
cms. and the total number of ions produced per second would be 
about 10% Now it has been shown in section 104 that an a 
particle projected from radium probably gives rise to 7 x 10* ions 
before it is absorbed in the gas. An expulsion of one a particle 
every 7 seconds from the whole vessel, or of one « particle from 
each square centimetre of surface per howr would thus account for 
the minute conductivity observed. Even if it were supposed that 
this activity 1s the result of a breaking up of the matter com- 
posing the vessel, the disintegration of one atom per second per 
gram, provided it was accompanied by the expulsion of an a 
particle, would fully account for the conductivity observed. 
Strutt? has recently observed that a radio-active emanation can 
be obtained by bubbling air through mercury. The emanation 
appears to be very similar in character to that emitted by radium 
emanation, for its activity decays to half value in 3:18 days and 
the excited activity to half value in 20 minutes. An emanation 
was also obtained by drawing air over red-hot copper. 
Bumstead and Wheeler” have repeated the experiments of 
Strutt of bubbling air through mercury, but were unable to 
detect any increase of the conductivity of air, which had been 
circulated through hot mercury for fourteen hours, although an 
increase of 10 per cent. of the natural conductivity could have 
been detected. These results indicate that the emanation from 
mercury obtained in the experiments of Strutt was probably due 
to the presence of a minute amount of radium as an impurity. 
There is not yet sufficient evidence to decide with certainty 
whether ordinary matter possesses the property of radio-activity. 
There is no doubt that, if matter possesses the property at all, 
it does so to a minute extent. The extreme minuteness of the 
1 Phil. Mag. July, 1903. 
2 Amer. Jour. Science, 17, p. 110, Feb. 1904. 
