DISCOVERY 



363 



is mostly space ; for the distances between the planets, 

 and between each planet and the sun, are enormous 

 compared with the size of the planets or of the sun. 

 It resembles it further in this, that very probably the 

 electrons revolve round the nucleus in the atom, like 

 planets round the sun. But the analogy need not be 

 pressed further. 



The amount of positive electricitj' on the nucleus of 

 the atom, and the number of electrons in the atom, 

 are invariably the same numerically. Thus, an atom 

 whose nucleus has a charge of 22 positive units has 

 22 electrons. The lead atom, for example, has 82 

 positive charges on its nucleus and 82 electrons ; the 

 oxygen atom 8 positive charges on its nucleus and 

 8 surrounding electrons ; the helium atom 2 and 2 ; 

 the hydrogen atom i and i. This number, which is 

 both the number of electrons in the atom and the 

 number of positive charges of electricity on the nucleus, 

 is an important one. It is called the Atomic Number. 

 Each element has one atomic number and one only ; 

 its chemical and physical properties depend upon this 

 magnitude, for they depend solely on the amount of 

 the charge on the nucleus. Change the atomic number 

 (if you can), and you change the chemical and physical 

 properties of the body ; that is, you make a new 

 element. 



I have tried to show, then, that atoms differ in weight 

 and chemical properties because of slight differences 

 in their general structure. These differences are 

 occasioned by differences in the number of positive 

 charges on the cores or nuclei of the different atoms, 

 and concurrently with this, in the number of electrons 

 which surround the nuclei. 



This important number, the Atomic Number, is ven,' 

 easily obtained. One simply arranges the whole of 

 the elements in increasing order of their atomic weights, 

 and numbers them off from the lowest like soldiers. 

 Hydrogen is the element of lightest atomic weight. 

 Its atomic number is therefore i, its nucleus has a 

 charge of i positive unit of electricity, it has i 

 electron. HeUum is the next Ughtest. Its atomic 

 number is therefore 2, its nucleus has a charge of 2 

 positive units, it has 2 electrons. Lithium is the third 

 lightest, hence, etc. Iron stands twenty-sixth in order 

 of lightness of atoms. It has therefore a charge of 

 26 positive units on its nucleus, and 26 electrons. The 

 statement applies to every element. Uranium, the 

 heaviest of the elements, stands ninety-second in 

 order. Its nucleus has 92 positive charges, and 

 round it are its 92 electrons. 



Note that it is not the number of electrons (each of 

 which is probably the same in size whatever atom it 

 occurs in) that makes the weight of the atom. The 

 electrons are supposed each to have a weight of only 

 TbVc of that of the lightest atom, so that their weight 



is negligible. The scat of the weight of the atom, as I 

 mentioned above, lies in the nucleus. Each positive 

 charge of electricity corresponds to from 2 to 2 J units 

 of atomic weight, depending upon whether the atom 

 be light or heavj-. Oxygen, for exampile, has an atomic 

 number of 8 and an atomic weight of 16 ; tin an 

 atomic number of 50 and an atomic weight of 119 ; 

 lead an atomic number of 82 and an atomic weight 

 of 207. 



The transmutation of the elements is a subject that 

 has alwa\'s appealed to the imagination of man. Many 

 people spent their valuable lives during the Middle 

 Ages attempting to make the noble metals out of the 

 baser sort. Seventeen years ago a firm in Scotland, 

 with a capital of between £50,000 and £100,000, tried 

 the experiment also as a side line to their proper work. 

 The present theory of the structure of the atom, had 

 they known it, would have convinced all these al- 

 chemists that the difficulty of transmutation was 

 greater than they may have imagined, even in their 

 most pessimistic moments. 



Gold has an atomic number of 79, mercury of 80. 

 To transmute mercury, which is comparatively cheap, 

 into an equal weight of gold, all one has to do, if the 

 present theory be true, is to reduce its atomic number 

 by one. To do this one has simply to knock out one 

 of the positive charges from the nucleus of mercury. 

 This will reduce the nucleus to 79 charges, and I am 

 quite sure the So surrounding electrons will then take 

 up a new set of positions, and expel one of their number 

 so that they too will number 79. The body with the 

 nucleus of 79 charges will be gold. No possible doubt 

 whate\-er about that. The great difficulty is to knock 

 out that single charge from the nucleus of the atom of 

 mercury. That is extremely difficult. Why ? Well, 

 first of all, you ha%-e to get hold of some tiny projectile 

 corresponding to the ball (now, alas, twopence !) that 

 brings down the cocoanut. This must be something 

 small because it has to penetrate the atom. A tiny 

 atom would do, helium or hydrogen for example. 

 Next, this small atom, in order to penetrate bodies, 

 must be moving with a considerable velocity, since 

 it is hitting the " outsides " of other atoms at every 

 stage and so losing velocity. The next difficulty is 

 that the nucleus is so small compared with the size of 

 the atom, being " mostly empty space," that only 

 one shot in many miUions is likely to hit the nucleus. 

 Most shots will go right through the atom and miss 

 the nucleus altogether. To be near is not enough. 

 The near shots are simply bent round the nucleus as a 

 comet is deflected round the sun. Once, however, the 

 nucleus is really hit with sufficient energy by the 

 projectile it is likely to be broken up. It will lose 

 some of its electric charge, and so will form with its 

 new set of surrounding electrons a different atom. 



