476 PRINCIPLES OF CHEMISTRY 



a stable position, such as occurs in pendulum-oscillations. A like periodic 

 function became evident in the case of the elements, depending on the mass 

 of the atom. The primary conception of the masses of bodies, or of the masses 

 of atoms, belongs to a category which the present state of science forbids us 

 to discuss, because as yet we have no means of dissecting or analysing the 

 conception. All that was known of functions dependent on masses derived 

 its origin from Galileo and Newton, and indicated that such functions 

 either decrease or increase with the increase of mass, like the attraction of 

 celestial bodies. The numerical expression of the phenomena was always 

 found to be proportional to 'the mass, and in no case was an increase of mass 

 followed by a recurrence of properties such as is disclosed by the periodic law 

 of the elements. This constituted such a novelty in the study of the phenomena 

 of nature that, although it did not lift the veil which conceals the true concep- 

 tion of mass it nevertheless indicated that the explanation of that conception 

 must be searched for in the masses of the atoms ; the more so, as all masses 

 are nothing but aggregations, or additions, of chemical atoms which would be 

 best described as chemical individuals. Let me remark, by the way, that 

 though the Latin word ' individual ' is merely a translation of the Greek word 

 atom,' nevertheless history and custom have drawn a sharp distinction 

 between the two words, and the present chemical conception of atoms is 

 nearer to that defined by the Latin word than by the Greek, although this 

 latter also has acquired a special meaning which was unknown to the classics. 

 The periodic law has shown that our chemical individuals display a harmonic 

 periodicity of properties dependent on their masses. Now natural science 

 has long been accustomed to. deal with periodicities observed in nature, to 

 seize them with the vice of mathematical analysis, to submit them to the> 

 rasp of experiment. And these instruments' of scientific thought wouldL 

 surely, long since, have mastered the problem connected with the chemical 

 elements, were it not for a new feature which was brpught to light by the 

 periodic law, and which gave a peculiar and original character to the periodic 

 function. 



If we mark on an axis of abscissa) a series of lengths proportional to 

 angles, and trace ordinates which are proportional to eines or other trigono- 

 metrical functions, we get periodic curves of a harmonic character. So it 

 might seem, at first sight, that with the increase of atomic weights the funct 

 tionof the properties of, the elements should also vary in the same harmonioui 

 way. But in this case there is no such continuous change as in th& curves 

 just referred to, because the periods do not contain the infinite number of 

 points constituting a curve, but a finite number only of such points. An 

 example will Better ffltfstrate this view. The atomic weights 



Ag = 108 Cd = 112 In = 113 Sn = 118 Sb-120 



Te = 125 1 = 127 



steadily 'increase, and their increase is accompanied by a modification of 

 many properties which constitutes the essence of the periodic law. Thus, 

 for example, the densities of the above elements decrease, steadily, being 

 respectively 



10-5 8-6 7'4 7'2 6-7 6'4 4-9 



