x] RADIO-ACTIVE PROCESSES 335 
hand it should be noticed that while the greatest amount of 
radium has been observed in a pitchblende rich in uranium, some 
pitchblendes rich in uranium contain very little radium. 
The general evidence, which has been advanced to show that 
radium must continually be produced from some other substance, 
applies also to actinium, which has an activity of the same order 
of magnitude as that of radium. It is very remarkable that the 
three radio-active substances, radium, thorium and actinium, should 
exhibit such a close similarity in the succession of changes which 
occur in them. Each of them at one stage of its disintegration 
emits a radio-active gas, and in each case this gas is transformed 
into a solid which is deposited upon the surface of bodies. It 
would appear that, after disintegration of an atom of any of these 
has once begun, there is a similar succession of changes, in which — 
the resulting systems have allied chemical and physical properties. 
Such a connection is of interest as indicating a possible origin 
of the recurrence of properties in the atoms of the elements, as 
exemplified by the periodic law. 
204. Loss of weight of the radio-element. Since the 
radio-elements are continuously throwing off a particles atomic 
in size, an active substance, enclosed in a vessel sufficiently thin to 
allow the a particles to escape, must gradually lose in weight. 
This loss of weight will be small under ordinary conditions, since 
the greater proportion of the a rays produced are absorbed in the 
mass of the substance. Ifa very thin layer of a radium compound 
were spread on a very thin sheet of substance, which did not 
appreciably absorb the @ particles, a loss of weight due to the 
expulsion of a particles might be detectable. Since e/m=6 x 10° 
for the « particle, and e=1-1 x 10-™ electro-magnetic units, and 
10" a particles are expelled per second per gram of radium, the 
fraction of the mass expelled is 1:8 x 10~* per second and 
6 x 10-* per year. There is one condition, however, under which 
the radium should lose in weight fairly rapidly. If a current of 
air is slowly passed over a radium solution, the emanation produced 
would be removed as fast as it was formed. Since the atom of 
the emanation has a mass probably not much smaller than the 
radium atom, the fraction of the mass removed per year should 
