DISCOVERY 



45 



fectly with the idea of atomic number, and it is now 

 perfectly straightfonvard to see how the number will 

 change in a transformation. For the emission of an 

 a-particle involves a loss of 2, and so the atomic number 

 will be reduced by 2 when it is thrown off. Similarly, 

 a y3-particle transformation will increase the number 

 by I, as here the particle is a negatively charged electron. 

 If this principle is applied the whole system of radio- 

 elements is cleared up, and a complicated chain can be 

 worked out, starting with uranium at 92, and ending 

 with lead at 82, some steps decreasing by 2 and others 

 increasing by i. A similar chain starts from thorium at 

 90, and alsoendswith lead. If these chains arc examined, 

 it will be found that most places are filled several times 

 over, but that none of the elements fall into the places 

 85 and Sj. These are the two other missing elements. 



We next push the principle of isotopes a httle 

 further. If the weights of all the particles it emits 

 during transformation are subtracted from the weight 

 of a uranium atom, the result should give the weight 

 of the end-product. There are 8 a-particles (the 

 /3-particles are insignificant in weight), and so the result 

 should be 2382 —8 x 4 = 2062. Now the end-product 

 is lead, and its atomic weight is usually accepted as 207' i. 

 This suggested determining the atomic weight of lead 

 derived from radio-active ores, as presumably this lead 

 would be partly composed of extinct radio-active matter, 

 and the value found was 2o6'4, quite different from 

 ordinary lead. This was the first definite attack on 

 the chemist's faith in atomic weights. This faith has 

 been shattered by the experiments now to be described. 



It has long been known that, if an electric discharge 

 goes through a vacuum tube (such a tube always con- 

 tains a little gas), charged atoms are produced moving 

 at high speeds, and so able to affect a photographic 

 plate. These are called positive rays, from the sign of 

 the charge they carry.' By means of electric and 

 magnetic fields they can be deflected, and from the 

 amount of deflection their weights can be found. 

 Working on these hues, Thomson examined the atomic 

 weights of many of the elements. In most cases the 

 result fully confirmed ordinary chemical e\-idence, but in 

 neon (atomic weight 20"2) it was consistently found that 

 there was a faint line on the photographs corresponding 

 to 22, as well as the line at 20. There was thus a strong 

 suggestion that neon is composed of two isotopes, 

 nine-tenths having an atomic weight of 20, and one- 

 tenth an atomic weight of 22. Now isotopes cannot be 

 separated by ordinary chemical means, yet any gases 

 whatever can be separated bj' the physical process of 

 diffusion if their weights are different. Aston there- 

 fore set to work to try and separate out a heavier type 

 of neon. The method is very tedious, even when made 



' An accoxmt of this subject will be found in Sir J. J. Thom- 

 son's Positive Rays. 



self-acting, as many million operations are needed. 

 After a long succession of trials he obtained faint 

 indication of improvement, but not enough to en- 

 courage him to proceed. He therefore turned his 

 attention again to positive rays, and with a greatly 

 improved apparatus was able to prove that neon is 

 certainly double, part exactly at 20 and part exactly 

 at 22. Turning to other substances, chlorine seemed of 

 special interest, as its atomic weight, 3546, is nowhere 

 near a whole number. He at once found that it is 

 double, three-quarters at 35 exact, and one-quarter at 

 37 exact. In the same sense mercury was also found 

 to be composite. These results are not two months 

 old, and there the matter rests for the present. 



It is impossible to see how far Aston's results will go, 

 but a little speculation is irresistible. It looks as if 

 we were going to find that all atomic weights are whole 

 numbers after all. This is simply a new form of a very 

 old idea, a forerunner of the periodic law, famous as 

 " Front's Hypothesis." The only difference is the im- 

 portant one that now an element may have more than 

 one atomic weight. In terms of Rutherford's theory, 

 this means that nuclei of the same total charge can be 

 built up in different ways exclusively out of hydrogen 

 atoms and electrons. For example, chlorine has number 

 17 and weight 35 or 37 ; so its nucleus is composed 

 either of 35 hydrogen atoms cemented together by 18 

 electrons, or 37 hydrogen atoms and 20 electrons. When 

 the corresponding facts have been explored for the 

 other elements, they may perhaps throw a dim light on 

 to the future problem of the structure of the nucleus. 



Summarising our results, we have shown how the 

 periodic law of the chemist has been replaced — it still 

 remains to explain it — by the conception of atomic 

 number — that is, the number of elementary charges of 

 positive electricity concentrated on the nucleus of the 

 atom. We have seen that, of the 92 elements from 

 hydrogen to uranium, all are known except five, Nos. 

 43. 61, 75, 85, and 87. Looking back to the periodic 

 table, we can say that 43 and 75 would bear a family 

 resemblance to manganese, 61 would be a rare earth, 

 85 a halogen (i.e. one of the chlorine group), and 87 an 

 alkali-metal. From another point of view we have 

 seen that atomic weight is only loosely connected with 

 atomic number, and it looks as though all nuclei were 

 built up of hydrogen atoms and electrons exclusively. 

 There is thus a curious inversion of the interests of 

 chemist and physicist. The reverence of the chemist 

 for atomic weight must cease, and in future he must 

 venerate atomic number only. On the other hand, an 

 exact knowledge of atomic weights will be one of the 

 few guides the physicist will have in his investigation 

 of the structure of the atomic nucleus, the main foun- 

 dation of all matter. 



