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 1 . Three 

 types of rays \vere 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 /3 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 /3-rays much more than a-rays, indicating that, although the 



1 Rutherford, Radio-activity (2nd edit.), Cambridge, 1905. 



