Ix] EXCITED RADIO-ACTIVITY 283 
along the lines of force in an electric field. For example, if a small 
negatively charged metal plate was placed in the centre of a metal 
vessel containing an emanating thorium compound, more excited 
activity was produced on the sides and corners of the plate than at 
the central part. 
A difficulty however arises in connection with the positive 
charge of the carrier. According to the view developed in sec- 
tion 127 and later in section 200, the matter emanation X, which 
is deposited on bodies and gives rise to excited activity, is itself 
derived from the emanation. The emanations of thorium and 
radium emit only a rays, ve. positively charged particles. After 
the expulsion of an a particle, the residue, which is supposed to 
constitute the emanation X, should retain a negative charge, and 
be carried to the anode im an electric field. The exact opposite is 
however observed to be the case. The experimental evidence does 
not support the view that the positively charged a particles, 
expelled from the emanation, are directly responsible for the 
phenomena of excited activity; for no excited activity is produced 
in a body exposed to the a rays of the emanation, provided the 
emanation itself does not come in contact with it. It may be 
supposed that in gases the matter emanation X, immediately after 
its production, attaches itself to the positive ions, produced in 
the gas by the radiation, on the same sort of principie that water 
vapour condenses round the negative ion. The active matter is 
then transported by these positive carriers to the cathode. In the 
case of radium, there is evidence that some of the carriers of 
excited activity do not acquire a positive charge until they have 
been present in the gas for some time. 
Whatever view is taken of the process by which these carriers 
obtain a positive charge, there can be little doubt that the expul- 
sion of an @ particle with great velocity from the atom of the 
emanation must set the residue in motion. On account of the 
comparatively large mass of this residue, which constitutes the 
emanation X, the velocity acquired will be small compared with 
that of the expelled a particle, and the moving mass will be 
rapidly brought to rest at atmospheric pressure by collision with 
the gas molecules in its path. At low pressures, however, the 
collisions will be so few that it will not be brought to rest until it 
