570 The Evolution of Matter 
microscope with a scale in the eye-piece, measures the intensity of 
the radiation. With some form of this simple instrument, or with 
the more complicated quadrant electrometer, most radio-active 
measurements have been made. 
It was soon discovered that the activity of uranium compounds 
was proportional to the amount of uranium present in them. Thus 
radio-activity is an atomic property dependent on the amount of an 
element and independent of its state of chemical combination. 
In a search for radio-activity in different minerals, M. and Mme 
Curie found a greater effect in pitch-blende than its contents of 
uranium warranted, and, led by the radio-active property alone, they 
succeeded, by a long series of chemical separations, in isolating com- 
pounds of a new and intensely radio-active substance which they 
named radium. 
Radium resembles barium in its chemical properties, and is pre- 
cipitated with barium in the ordinary course of chemical analysis. 
It is separated by a prolonged course of successive crystallisation, the 
chloride of radium being less soluble than that of barium, and there- 
fore sooner separated from an evaporating solution. When isolated, 
radium chloride has a composition, which, on the assumption that 
one atom of metal combines with two of chlorine as in barium 
chloride, indicates that the relative weight of the atom of radium 
is about 225. As thus prepared, radium is a well-marked chemical 
element, forming a series of compounds analogous to those of 
barium and showing a characteristic line spectrum. But, unlike 
most other chemical elements, it is intensely radio-active, and 
produces effects some two million times greater than those of 
uranium. 
In 1899 E. Rutherford, then of Montreal, discovered that the 
radiation from uranium, thorium and radium was complex). Three 
types of rays were soon distinguished. The first, named by Rutherford 
a-rays, are absorbed by thin metal foil or a few centimetres of air. 
When examined by measurements of the deflections caused by 
magnetic and electric fields, the a-rays are found to behave as would 
positively electrified particles of the magnitude of, helium atoms 
possessing a double ionic charge and travelling with a velocity about 
one-tenth that of light. The second or 8 type of radiation is much 
more penetrating. It will pass through a considerable thickness of 
metallic foil, or many centimetres of air, and still affect photographic 
plates or discharge electroscopes. Magnetic and electric forces 
deflect 8-rays much more than a-rays, indicating that, although the 
* Rutherford, Radio-activity (2nd edit.), Cambridge, 1905. 
