
224, RADIO-ACTIVE EMANATIONS 
that the emanation does not give rise to y rays; these rays always 
make their appearance at the same time as the # rays. 
The method of examination of the radiations from the 
emanations has been given in some detail, as the results are of 
considerable importance in the discussion, which will be given 
later in chapter X, of the connection between the changes oc- 
curring in radio-active products and the radiations they emit. 
There is no doubt that the emanations, apart from the excited 
activity to which they give rise, only give out @ rays, consisting 
most probably of positively charged bodies projected with great 
velocity. 
Effect of Pressure on the rate of production of the Emanation. 
148. It has already been mentioned that the conductivity 
due to the thorium emanation is proportional to the pressure of 
the gas, poimting to the conclusion that the rate of production 
of the emanation is independent of the pressure, as well as of the 
nature of the surrounding gas. This result was directly confirmed 
with the apparatus of Fig. 41. When the pressure of the gas 
under the vessel was slowly reduced, the radiation, tested outside 
the window, increased to a limit, and then remained constant 
over a wide range of pressure. This increase, which was far more 
marked in air than in hydrogen, is due to the fact that the a rays 
from the emanation were partially absorbed in the gas inside the 
vessel when at atmospheric pressure. At pressures of the order 
of 1 millimetre of mercury the external radiation decreased, but 
experiment showed that this must be ascribed to a removal of the 
emanation by the pump, and not to a change in the rate of pro- 
duction. The thorium compounds very readily absorb water-vapour, 
which is slowly given off at low pressures, and in consequence 
some of the emanation is carried out of the vessel with the water- 
vapour. 
Curie and Debierne! found that both the amount of excited 
activity produced in a closed vessel containing active samples of 
radium, and also the time taken to reach a maximum value, were 
1 GC. R. 133, p. 931, 1901. 
