vill] RADIO-ACTIVE EMANATIONS 243 
159. Thus there is no doubt that the thorium emanation 
begins to condense at a temperature higher than that at which 
the radium emanation condenses. The explanation of the pecu- 
har behaviour of the thorium emanation is clear when the small 
number of emanation particles present in the gas are taken into 
consideration. It has been shown that both emanations give 
out only a rays. It is probable that the a particles from the 
two emanations are similar in character and produce about the 
same number of ions in their passage through the gas. The 
number of ions produced by each a particle before its energy 
is dissipated is probably about 70,000. (See section 104.) 
Now in the experiment the electrometer readily measured 
a current of 10~* electrostatic units. Taking the charge on an ion 
as 3:4 x 10” electrostatic units, this corresponds to a production in 
the testing vessel of about 3 x 10° ions per sec., which would be 
produced by about 40 expelled a particles per second. Each 
radiating particle cannot expel less than one a particle and may 
expel more, but it is likely that the number expelled by an atom 
of the thorium emanation is not greatly different from the number 
by an atom of the radium emanation. 
In section 124 it has been shown that, according to the law of 
decay, XN particles change per second when WN are present. Thus 
to produce 40 a particles, AN cannot be greater than 40. Since for 
the thorium emanation 2 is 1/87, it follows that N cannot be greater 
than 3500. The electrometer thus detected the presence of 3500 
particles of the thorium emanation, and since in the static method 
the volume of the condensing spiral was about 15 c.c., this corre- 
sponds to a concentration of about 2380 particles per cc. An 
ordinary gas at atmospheric pressure and temperature probably 
contains about 3°6 x 10" molecules per c.c. Thus the emanation 
would have been detected on the spiral if it possessed a partial 
pressure of less than 10 of an atmosphere. 
It is thus not surprising that the condensation point of the 
thorium emanation is not sharply defined. It is rather a matter 
of remark that condensation should occur so readily with so sparse 
a distribution of emanation particles in the gas; for, in order 
that condensation may take place, it is probable that the particles 
must approach within one another’s sphere of influence. 
16—2 
