en 
Cy are < 
ee 
190 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 
duct at any tume is proportional to the number of atoms which 
remain unchanged at that trme. 
This is the same as the law of mono-molecular change in 
chemistry, and expresses the fact that there is only one changing 
system. If the change depended on the mutual action of two 
systems, the law of decay would be different, since the rate of 
decay in that case would depend on the relative concentration 
of the two reacting substances. This is not so, for there is not 
a single case yet observed in which the law of decay was affected 
by the amount of active matter present. 
From the above equation (1) 
dN; 
Sagar Ome rN, 
or the number of systems changing in unit time is proportional to 
the number unchanged at that time. 
In the case of recovery of activity, after an active product has 
been removed, the number of systems changing in unit time, when 
radio-active equilibrium is produced, is equal to AN,. This must 
be equal to the number gq, of new systems supplied in unit time, or 
Cy; rN, 
and eos 
and N,: 
» has thus a distinct physical meaning, and may be defined as 
the proportion of the total number of systems present which 
change per second. It has a different value for each type of 
active matter, but is invariable for that particular type of matter. 
For this reason, X will be termed the “vradio-active constant” of 
the product. 
125. Influence of conditions on the rate of decay. 
Simce the activity of any product, at any time, may be taken as 
a measure of the rate at which chemical change takes place, 16 
may be used as a means of determining the effect of conditions 
on the changes occurring in radio-active matter. If the rate of 
change should be accelerated or retarded, it is to be expected 
that the value of the radio-active constant 2 would be increased or 
