Vil] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 195 
atoms of Th X break up in four days, the number breaking up 
per second being always proportional to the number present. 
After an atom of Th X has expelled an @ particle, the mass of the 
system is again reduced and its chemical properties are changed. 
It will be shown (section 145) that the Th X gives rise to the 
thoriuyjp emanation, which exists as a gas, and that this in turn 
gives rise to matter which is deposited on solid bodies and gives 
rise to the phenomena of excited activity. 
Asa result of the disintegration of the thorium atom, there is 
thus a series of chemical substances produced, each of which has 
distinctive chemical properties. Each of these products is radio- 
active, and loses its activity according to a definite law. Since 
thorium has an atomic weight of 237, and the weight of the 
a particle is about 2, it is evident that, if only one a particle 
is expelled at each change, the process of disintegration could 
pass through a number of successive stages and yet leave behind, 
at the end of the process, a mass comparable with that of the 
parent atom. 
It will be shown in chapter X that a process of disintegration, 
very similar to that already described for thorium, must be sup- 
posed to take place also in uranium and radium. The full 
discussion of this subject cannot be given with advantage until 
two of the most important products of thorium and radium, viz. 
the radio-active emanations and the matter which causes excited 
activity, have been considered in detail. 
128. Magnitude of the changes. It can be calculated 
by several independent methods that, in order to account for the 
changes occurring in thorium, probably not more than 10° and 
not less than 10* atoms in each gram of thorium suffer disintegra- 
tion per second. It is well known (section 39) that 1 cubic centi- 
metre of hydrogen at atmospheric pressure and temperature contains 
about 2 x 10" molecules. From this it follows that one gram of 
thorium contains about 107 atoms. The fraction which breaks 
up per second thus les between 10-” and 10-%, This is an 
extremely small ratio, and it is evident that the process could 
continue for long intervals of time, before the amount of matter 
changed would be capable of detection by the spectroscope or 
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