THE CONSTITUTION OF MATTER 125 



radium C. In the transformation of the latter, not only are swift 

 alpha rays emitted but also beta rays of great speed. There is some evi- 

 dence, however, that the substance called radium C is complex, and that 

 the alpha and beta rays arise from two distinct substances. 



The successive substances arising from radium C are radium D, 

 radium E and radium F. The two former, like radium B, emit only 

 beta rays; the latter, known generally as polonium, emits only alpha 

 rays. It is believed that the sequence of changes ends with the trans- 

 formation of radium F, which is supposed to change into the well- 

 known non-radioactive element lead. 



According to the transformation theory radium, like all other radio- 

 active products, must be regarded as a changing element, but one whose 

 rate of transformation is very slow compared with its successive 

 products. Boltwood showed experimentally that radium is half trans- 

 formed in about 2,000 3'ears, and a quantity of radium would prac- 

 tically have disappeared as such in 100,000 years. In order to account 

 for the continued existence of radium in the earth, it is necessary to 

 suppose that it is steadily produced from some other element. Bolt- 

 wood showed that the parent substance is a radioactive element called 

 ionium, which is itself derived from the transformation of uranium. 

 A quantity of ionium, entirely freed from radium, will grow radium 

 at a slow but constant rate. The primary element of the ionium- 

 radium series is uranium, which we can calculate should be half trans- 

 formed in 5,000 million years — a period probably long compared with 

 the age of many of the minerals in which uranium is found. 



The complete sequence of changes in the uranium-radium series is 

 shown in the diagram. The nature of the radiation and the half period 

 of transformation are added for each element. In addition to ura- 

 nium, there are two other radioactive elements, thorium and actin- 

 ium, which are transformed with the appearance of a number of new 

 substances. The time at my disposal, however, is too short to discuss 

 these changes in detail. Thorium is known to be a primary element 

 whose radioactive life is even longer than uranium, but actinium is be- 

 lieved to be a branch descendant from some point of the uranium series, 

 and is thus to be regarded as a product of that element. In all, thirty- 

 four of these radioactive substances have been discovered, and the 

 position of each in the three main radioactive series has been 

 determined. 



Each of these new substances is to be regarded as a distinct chemical 

 element in the ordinary sense, but differs from ordinary stable elements 

 in the spontaneous emission of special radiations which accompanies 

 the disintegration of the atoms. The radioactive substances are thus 

 transition elements which have a limited life and which carry within 

 themselves the seeds of their own destruction. Not only are these transi- 

 tion elements distinguished by their types of radiation but also bv 



