x] RADIO-ACTIVE PROCESSES 315 
rays are material in nature. The mass and velocity of the « 
particles, projected from radium, have been measured. They have 
been shown to be positively charged particles projected with a 
velocity of about 1/10 the velocity of light, and a mass about 
twice that of the hydrogen atom. The @ particles carry a negative 
charge, and have an apparent mass about 1/1000 the mass of the 
hydrogen atom. They are identical in character with the cathode 
ray particle produced in a vacuum tube. The nature of the y rays 
has not yet been determined. 
The « rays play by far the most important part in radio-active 
processes. Most of the energy radiated in the form of ionizing rays 
is due to them. In addition, most of the active products emit only 
arays. The 8 and y rays in most cases only appear in the last 
stage of the radio-active processes. 
The activity of most of the products decays according to an 
exponential law with the time. In cases where this does not hold, 
the activity can be shown to be due to several successive changes, 
the rate of each of which decays according to an exponential law 
but with a different radio-active constant. The rate of decay of 
activity has not yet been found to be in any way influenced by 
wide variation in chemical and physical conditions. 
The activity of any product at any time (section 124), is 
proportional to the rate of change of the product, and 1s also pro- 
portional to the amount of matter left unchanged. In cases where 
one active product gives rise to another, the activity of the first 
product is, at any time, a measure of the rate of production of the 
second product. In other words, the radiations accompany the 
change of one product into another, and serve as a measure of the 
rate of change. This point of view at once follows if the expulsion 
of rays is taken to be the cause of the change from one product 
into another. The rate of emission of @ particles is a measure of 
the rate of change of the first product, i.e. it is proportional to the 
rate at which the second product is produced. For example, the 
amount of emanation X of thorium produced in a given time by 
the thorium emanation is proportional to the activity of the ema- 
nation. In cases where the rate of change of the second product is 
rapid compared with that of the first, after sufficient interval has 
elapsed in order to reach a state of approximate radio-active equi- 
