96 



DISCOVERY 



therefore has a negligibly small mass), which is expelled 

 with a velocity which varies from about 50,000 miles 

 per second to that of light itself. A radio-element 

 lacks no property which is characteristic of an ordinary 

 element. Its properties are exactly those to be ex- 

 pected from its atomic weight, and are indeed those 

 it would have, were it not radio-active. The pro- 

 perties which are associated with the name " radio- 

 active " are additional properties, extras. Lack of 

 permanence, as we shall see, is one of these. 



The study of radio-activity in the last twenty years 

 has supplied experimental proof of the individual 

 existence of the atom as a real unit in the structure 

 of matter. (It is true that for more than a hundred 

 years chemists have found it extremely convenient 

 to postulate the existence of atoms, but it is only 

 in our own day that the physicists have been able to 

 demonstrate that atoms really do exist.) An element 

 therefore consists of an assemblage of atoms. A radio- 

 element differs from one which is not radio-active in 

 that in a given interval of time, say a second, a definite 

 fraction of the total number of its atoms happen to 

 disintegrate. Each disintegrating atom expels either 

 an a- or a /3-particle and becomes in consequence an 

 atom different from what it was prior to the act of 

 disintegration, or, what is the same thing, from an 

 atom that has not disintegrated. A radio-element 

 consequently contains always two kinds of atoms, 

 those that have disintegrated, and those that have 

 not. Those that are unchanged comprise the pure 

 radio-element which is known as the parent ; those 

 that have disintegrated comprise a new element known 

 as the product. The product is perfectly distinct from 

 its parent in phy.sical and chemical properties, and can 

 be easily separated from it by the ordinary methods 

 of analytical chemistry. If now the product happens, 

 like its parent, to be radio-active, a certain fraction 

 of it will disintegrate per second to form its prcduct, 

 a third substance, and this body, if radio-active, will 

 produce a fourth, the fourth a fifth, and so on, till a 

 body is reached which has not the power of disin- 

 tegrating, when the series of elements abruptly ends. 

 Such a series is called a disintegration series, and three 

 of these are known at the present time. In a dis- 

 integration series each element but the last is the 

 parent of the one that follows, and, except the first, 

 the product of the one that precedes. The first body, 

 the head of the series, is called 3l primary radio-element, 

 or sometimes the original parent. Uranium and 

 thorium are two of the three primary radio-elements. 



It must be pointed out that a radio-active body 

 never disintegrates " all at once." The process may 

 proceed quickly or slowly according to the properties 

 of the element disintegrating, but it always proceeds 

 according to one settled plan. In a given interval of 



time there is always a definite fraction of the atoms of 

 each radio-element which disintegrate, and this frac- 

 tion is invariable. It is the same whether there be a 

 milhon million atoms present or a million only ; no 

 chemical combination with other atoms — or physi- 

 cal agency such as change of pressure or of temperature 

 — seems able to affect the value of this fraction in the 

 least degree. The fraction for the best known radio- 

 element, radium, is ^^3^9 ?£•" year. This means that 

 if we were to weigh out 2,309 pounds (or any other 

 unit of weight) of radium to-day, in a year's time 2,308 

 would remain absolutely unchanged and i would 

 represent the weight of products into which the radium 

 had disintegrated. Expressed otherwise, 99'95 per 

 cent, (if you work it out) of the radium fails to show 

 any sign of radio-activity in the course of a year, so that 

 the radio-activity of radium is due to a very small 

 percentage of itself. Most radio-elements disintegrate 

 more rapidly than radium ; five disintegrate more 

 slowly. 



Let us now consider the matter more in detail. 

 Suppose a body disintegrates so rapidly that time may 

 be conveniently reckoned in days instead of in years. 

 We shall imagine that the fraction which disintegrates 

 each day is 10 per cent. It must not be deduced from 

 this that the whole of the body wiU consequently dis- 

 appear in ten days. The percentage that disintegrates 

 each day is calculated on the actual number of atoms 

 of the radio-active body in existence at the beginning 

 of that day. Let there be 1,000,000 atoms at any 

 particular time, then exactly a day later there wiU be 

 900,000, after two days 810,000, after three days 

 729,000, after four days 656,100, after five days 590,490, 

 after six days 531,441, and after a week 478,297. 

 Each of these numbers is 10 per cent, less than the 

 number which precedes it. All radio-elements, when 

 pure, disintegrate according to a scheme of this kind. 

 The fraction that disintegrates daily varies enonnously 

 for the different elements, but for each element, as has 

 been said, it is invariable. For equal intervals of 

 time (in the example, one day) the ratio of the number 

 of atoms of the original radio-element at the end of 

 the interval to that at the beginning is constant (in 

 the example y^). This is the law of disintegration 

 which governs all known radio-active changes. It is 

 sometimes stated in this form : as time increases in 

 arithmetical progression, the number of atoms decreases 

 in geometrical progression. In Fig. i the type of curve 

 obtained by plotting the number of atoms (or the 

 activity) against the time for the radio-element uranium 

 Xis shown. It is known as a decay-curve. 



There are two interesting consequences of this law 

 which are not ob\dous, but which will nevertheless 

 be mentioned. The first is that, although the number 

 ol atoms comprising the radio-element change with 



