RADIUM AND THE ELECTRON—RUTHERFORD. 901 
matter. The electric method devised by Rutherford and Geiger of 
counting single «-particles allows us to count the total number 
of a-particles projected from one gram of radium per second. By 
determining the volume of helium produced by the collected a- 
particles, we have a simple and direct method of determining 
also the number of molecules in a cubic centimeter of helium at 
standard pressure and temperature. This number is in good agree- 
ment with the number found by Millikan by measuring the charge 
on the atom of electricity. On account of the great energy of motion 
a single «-particle can be detected in a variety of ways by the elec- 
trical method, by the scintillations produced in zinc sulphide or the 
diamond, and by its action on a photographic plate. 
The most striking proof of the individuality of the electron, the «- 
particle, and the ion has been given by C. T. R. Wilson by his beauti- 
ful photographs showing the trails of «- and @-particles through gases. 
By a sudden expansion each charged ion produced by the flying 
particle is rendered visible by becoming the center of a visible drop 
of water. In the case of the swift electron, the number of ions per 
centimeter of path is so small that the number may be directly 
counted. These photographs bring out in a vivid and concrete way 
the phenomena accompanying the passage of ionizing types of radia- 
tion through gases, and are in a sense the ultimate court of appeal 
of the accuracy of theories of the properties of these rays. 
The discovery of the electron and of the property of radioac- 
tivity has given a great stimulus to attempts to deduce the structure 
of the atom itself, and numerous types of model atoms have been 
proposed. The great difficulty in these attempts is the uncertainty 
of the relative importance of the réle played by positive and negative 
electricity. In the model atom proposed by Sir J. J. Thomson the 
electrons were supposed to be embedded in a sphere of positive elec- 
tricity of about the dimension of the atom as ordinarily understood. 
Experiments on the scattering of a-particles through large angles as 
the result of a single collision with a heavy atom showed that this 
type of atom was not capable of accounting for the facts unless the 
positive sphere was much concentrated. This led to the nucleus atom 
of Rutherford, where the positive charge and also the mass of the 
atom are supposed to be concentrated on a nucleus of minute dimen- 
sions. The nucleus is surrounded at a distance by a distribution of 
negative electrons to make it electrically neutral. The distribution 
of the external electrons on which the ordinary physical and chemical 
properties of the atom depend is almost entirely governed by the mag- 
nitude of the positive charge. The experiments by Marsden and 
Geiger on the scattering of the e-particles, and also on the scattering 
of X-rays by Barkla, show that the resultant units of charge on the 
