THE PRESENT CONCEPTION OF MATTER 79 



and therefore, as we shall see, the number of electrons in the 

 atom, is equal to about half the atomic weight of the substance 

 considered. 



But what do we mean by this term " atomic weight " ? 

 We mean, in effect, the weights of the atoms of the elements, 

 these being different as we pass from element to element. 

 They cannot, of course, be measured in the straightforward 

 way of weighing the atoms singly in turn ; they are determined 

 indirectly by methods based on the fundamental facts of 

 chemistry. Dalton showed, many years ago, that when 

 chemical change occurs between two elements it occurs in 

 definite proportions by weight of the two elements. If unit 

 weight of one is taken, then the weight of the other is a definite 

 and fixed amount. If the same elements combine to form 

 other compounds in other proportions, then if unit weight of 

 the first be taken again, the weight of the second will still be 

 some simple multiple or sub-multiple of the first. This indicates 

 that the combination of elements does not occur in a haphazard 

 way, but atom by atom by definite relative weight. So we get 

 what we call the atomic weights. 



When they have been found for the different elements, their 

 values are expressed not as very small fractions of a very small 

 weight, such as an ounce or grain or gramme, but simply as 

 numbers on a scale. It must not be forgotten that although 

 these numbers are called " atomic weights," they are not the 

 actual weights of the atoms, but are for purposes of comparison 

 only. The standard of comparison is the atomic weight of 

 oxygen, which is chosen as being 16. Uranium, the heaviest 

 element, is then 238, gold is 197, zinc is 65, helium 4, and 

 hydrogen just a little more than i. So we know at once the 

 comparative weights of the atoms of the elements, and we can 

 easily arrange them in ascending order. 



The question was first tackled by Newlands in 1 864, but was 

 developed further five years later by Mendeleeff and Meyer 

 independently. When the elements are written down in the 

 proper order of their atomic weights, it is seen that consecutive 

 ones differ from each other in general properties and charac- 

 teristics in a most abrupt and surprising way, much (to use an 

 analogy given by Professor Soddy) as do the notes in a scale of 

 music. But when several of the elements have been passed, 

 it is found that their general characteristics are reproduced by 

 the next set, in much the same way as the next octave of the 

 musical scale reproduces the general characteristics of the 

 preceding one. When Newlands formulated his law it so 

 happened that the number of elements known was such that 

 this reproduction of characteristics began with the eighth 

 element after the first, and hence he called it the Law of Octaves. 



