DISCOVERY 



201 



the nucleus like planets round the sun are electrons, 

 which are single charges of negative electricity and are 

 of negligible mass. The number of electrons is exactly 

 the same as the net number of positive charges on the 

 nucleus. This number is called the Atomic Number. 



4. Each chemical element has one atomic number 

 only. With each atomic number one element only is 

 associated. Hydrogen, whose atom is the lightest 

 known, has the atomic number i ; uranium, whose 

 atom is the heaviest known, has the atomic number 92. 

 Consequently there can be only 92 different chemical 

 elements in e.xistence. Of these five alone remain to 

 be discovered. 



In this article I propose to deal more particularly with 

 groups of chemical elements which are called isotopes' 

 and to explain how the experimental work which led to 

 the discovery of these bodies and the theoretical ideas 

 which have been put forward to fit them into the 

 existing scheme of things have led to a remarkable 

 simplification in our knowledge of what the chemical 

 elements are. 



At the present moment we define elements as sub- 

 stances which cannot be decomposed into simpler sub- 

 stances by any chemical process. Nobody ever denies 

 that theoretically further decomposition is possible, but 

 nobody has ever met anyone who has effected the 

 decomposition, so there the matter ends. Because the 

 element could not be resolved further chemically it was 

 quite natural for chemists to conclude that the reason 

 for this was that all the atoms composing the element 

 were exactly similar, for if they were not some ingenious 

 investigator would probably have discovered the fact, 

 and worked out a method of separation based upon it. 

 Yet although chemical elements still cannot be decom- 

 posed by chemical means into simpler substances it is 

 not a fact that the atoms composing a chemical element 

 are necessarily all the same. From two independent 

 lines of investigation, namely, the study of radio- 

 activity and what is called positive-ray analysis, we 

 now know that a great many (probably, when the work 

 now in progress is completed, the majority) of the 

 chemical elements are really mixtures of two, three, and 

 sometimes more than three elements. 



The mixture of elements which comprise one of these 

 elements is, as far as we know, uniform in composition 

 all over the world, wherever it be found or however it 

 be prepared. The constituents cannot be separated one 

 from another by any ordinary process, because they 

 have extraordinarily similar chemical properties. 

 They differ in two important physical properties- — in 

 their atomic weights (and properties depending upon 

 atomic weight) and (such as possess it) in their radio- 



' So called because in the periodic system of classification 

 of elements they occupy the same placfe. (Greek, tVoi, same, 

 TOTos, place.) They are defined later. 



activity. In the remainder of their physical properties, 

 for instance, in their arc-spectra, their X-raj' spectra, 

 their potentials, their boiling-points, freezing-points, 

 etc., they are extraordinarily identical. It is because 

 of these two differences that the separate existence of 

 these very similar elements has been established, for 

 otherwise it would not have occurred to anyone to sus- 

 pect that what appeared simple was in reality complex. 

 These similar elements are called isotopes. 



An account of the element chlorine ma\' help to make 

 the above clearer. Chlorine is a substance easily 

 isolated and purified, and of an undeniable elementary 

 character. Its atomic weight has been often accurately 

 determined, and has been found to be 35'46. Yet 

 chlorine is a mixture of two elements whose similarity 

 in every chemical and physical propert}' e.xcept that of 

 atomic weight is an identical one. The atomic weight 

 of one is 35, of the other 27, and what we call chlorine 

 is a mixture of uniform composition of these two ele- 

 ments in the ratio of approximately 4 to i. 



Note that these two elements which together com- 

 pose the element as we know it are elements in every 

 sense of the word. There is no need to coin a word 

 like sub-element, for that word would suggest that they 

 were merely parts of an element. Note also that the 

 element as we know it is not a compound but merely 

 a mixture of the two elements. It also is an element, 

 since by definition it is a substance which cannot be 

 split up into simpler substances. 



The history of these very similar chemical elements, 

 or isotopes as they are now called, is not without inter- 

 est. They were discovered by chemists in carrying 

 out research in radio-activity, but at the present time 

 the chief work on the subject is being done by physicists . 

 In 1906 Professor Boltwood of Yale noticed that an 

 element discov'ered by him and named ionium was simi- 

 lar in its chemical properties to the element thorium. 

 These were quite separate and distinct elements, of 

 course, but he found that if compounds of these two ele- 

 ments got mixed in the course of his work the tw-o kept 

 together, and did not allow themselves to be separated 

 by any chemical process he had occasion to put them 

 through. There is nothing very remarkable in this 

 fact, for there are several groups of elements which are 

 well known to be separable only by a long and tedious 

 process. Copper is easily separated from iron. Iron 

 is less easily separated from aluminium. Neodymium 

 is difficultly separable from praesodymium, and the 

 separation of other members of w hat are called the rare 

 earths from one another is a matter of greater difficulty 

 still. The question now arises : Is the separation of 

 ionium from thorium just about as difficult as some of 

 these rare-earth separations, or is it very much more 

 difficult ? Boltwood did not pursue the subject. He 

 had other work in hand, more interesting, and at the 



