x] RADIO-ACTIVE PROCESSES 309 
#1 —e-% where ® is the same constant as before. It thus 
0 
follows that 7, -+,=1,, which is an expression of the above result. 
The same is also true whatever the law of decay of activity of the 
separated product (see section 174). For example, the activity of 
Th X after separation from thorium at first increases with the 
time. At the same time, the activity of the residual thorium 
compound at first decreases, and at such a rate, that the sum of 
the activities of the thorium and its separated product is always 
equal to that of the original thorium. 
This principle of “ conservation of radio-activity’,’ as it may be 
termed, follows from the general result that the radio-active pro- 
cesses cannot in any way be changed by the action of known 
forces. It may be recalled that the rate of decay of the activity of 
a radio-active product cannot be altered by any known agency. 
The rate of decay is independent of the concentration of the active 
matter, of the pressure and of the nature of the gas in which it is 
placed, and is not affected by wide ranges of temperature. In the 
same way, 1t has not been found possible to alter the rate of 
production of active matter from the radio-elements. In addition, 
there is not a single case yet observed where radio-activity has 
been altered or destroyed in any active body or created in an 
inactive element. 
Certain cases have been observed, which at first sight seem to 
indicate a destruction of radio-activity. For example, the excited 
radio-activity is removed from a platinum wire when heated above 
a red heat. It has been shown, however, by Miss Gates (sec- 
tion 180) that the radio-activity is not destroyed, but is deposited 
in unaltered amount on the colder bodies surrounding it. Thorium 
oxide has been shown to lose its power in emanating to a large 
extent by ignition to a white heat. But a close examination shows 
that the emanation is still being produced at the same rate, but is 
occluded in the compound. 
The total radio-activity of a given mass of a radio-element, 
measured by the peculiar radiations emitted, is a quantity which 
can neither be increased nor diminished, although it may be mani- 
fested in a series of products which are capable of separation from 
1 Rutherford and Soddy, Phil. Mag. May, 1903. 
