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IOWA ACADEMY OP SCIENCE 
at all times multitudes of minute dust particles. When such air is admitted 
into the fog chamber 'saturated with vapor, the dust particles on exhaustion be- 
come the nuclei of small droplets of water which taken together constitute the 
cloud. As Barus states, such particles induce the condensation of vapor. These 
dust like nuclei are persistent. By the term “persistent” used so much by 
Barus, is meant that they will remain suspended in the gas of the fog chamber 
indefinitely until an exhaustion is made. Several successive exhaustions, how- 
ever, will bring them down in droplets and thus purify the air. These per- 
sistent nuclei are particularly efiicient for nucleation at a difference of pressure 
of 16 to 18 cm. By difference of pressure is meant the difference between the 
barometric pressure in the fog chamber produced by exhaustion and that of the 
outside atmospheric pressure. 
The second class of nuclei denoted by Barus are called “fleeting nuclei.” 
They are produced by some ionizing source, such as X-rays or radium. “They 
carry a charge of electrification and are called ions.” These fleeting nuclei are 
most efficient for nucleation at a dp. of 19 cm. or higher. When a weak radium 
compound is brought near the fog chamber, the interior is instantly surcharged 
■with fleeting nuclei. Barus does not attempt to describe how these are produced. 
The experimental work of Rutherford has abundantly proven that the Alpha and 
Beta rays of a radio active ppmpound are the principal sources of its ionizing 
power when they are not intercepted by some intervening solid. The Alpha rays 
are by far the best ionizers, but unfortunately they are easily cut off by the 
slightest obstruction, such as a thin piece of celluloid. It is evident then that 
the Alpha or Beta rays are ineffective as ionizers of the interior gas of a fog 
chamber having such thick walls of glass or wood as that used by Barus. Barus 
rightly concluded that the interior of his fog chamber was ionized wholly by the 
Gamma rays and their secondary effects on the walls of the vessel. The 
Gamma rays of radium, as we know, are like the X-rays, although decidedly 
more penetrating and more uniform in intensity. According to modern theory, 
the ionization of a gas by means of X-rays or Gamma rays is due to the fact 
that these rays have the power of disintegrating its atomic structure. A stable 
atom consists of a multitude of negatively charged electrons moving with high 
velocities inside of a positively charged sphere, the whole in static equilibrium. 
The action of the Gamma rays is to set free a negative electron leaving a sur- 
plus positive charge on the atom. Such a freed electron and its corresponding 
positively charged atom from which it has been delivered constitute the fleeting 
nuclei mentioned by Barus. They are fleeting because when the ionizing source, 
radium or X-rays, is removed, they at once recombine and disappear in very 
small intervals of time. 
Barus found also that persistent nuclei similar to those of a dust-like char- 
acter mentioned first could be induced by X-rays even in thoroughly dust free 
air. These persistent nuclei varied in number with the nature of the solid ma- 
terial thru which the X-rays past. They are probably disintegrated particles of 
matter carrying one or several ions. Like the dust nuclei, they produce large 
droplets on exhaustion at small differences of pressure. Those who have read 
the account of Wilson’s experiments to determine the charge of an electron by 
means of X-rays will remember that his clouds broke up into a succession of 
layers. Barus’ notion of persistent nuclei evoked by X-rays gives a simple and 
reasonable explanation of these layers. 
