Vi] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 191 
decreased, v.e. that the decay curve would be different under 
different conditions. 
No such effect, however, has yet been observed in any case of 
radio-active change, where none of the active products produced 
are allowed to escape from the system. The rate of decay is 
unaltered by any chemical or physical agency, and in this respect 
the changes in radio-active matter are sharply distinguished from 
ordinary chemical change. For example, the rate of decay of 
activity from any product takes place at the same rate when the 
substance is exposed to light as when it is kept in the dark, at 
the same rate in a vacuum as in air or any other gas at atmo- 
spheric pressure. Its rate of decay is unaltered by surrounding 
the active matter by a thick layer of lead under conditions where 
no ordinary radiation from outside can affect it. The activity of 
the matter is unaffected by ignition or chemical treatment. The 
material giving rise to the activity can be dissolved in acid and 
re-obtained by evaporation of the solution without altering the 
activity. The rate of decay is the same whether the active 
matter 1s retained in the solid state or kept in solution. When 
a product has lost its activity, resolution or heat does not re- 
generate it, and as we shall see later, the rate of decay of the 
active products, so far examined, is the same at a red heat as at 
the temperature of liquid air. In fact, no variation of physical or 
chemical conditions has led to any observable difference in the 
decay of activity of any of the numerous types of active matter 
which have been examined. 
126. Effect of conditions on the rate of recovery of 
activity. The recovery of the activity of a radio-element with 
time, when an active product is separated from it, 1s governed by 
the rate of production of fresh active matter and by the decay of 
activity of that already produced. Since the rate of decay of the 
activity of the separated product 1s independent of conditions, the 
rate of recovery of activity can be modified only by a change of 
the rate of production of fresh active matter. As far as experi- 
ments have gone, the rate of production, like the rate of decay, is 
independent of chemical or physical conditions. There are indeed 
certain cases which are apparent exceptions to this rule. For 
