172 PRINCIPLES OF CHEMISTRY 



The striking observations with the spectroscope which have permitted us to 

 analyse the chemical constitution of distant worlds, seemed, at first, appli- 

 cable to the task of determining the nature of the atoms themselves ; but the 

 working out of the idea in the laboratory soon demonstrated that the charac- 

 ters of spectra are determined, not directly by the atoms, but by the mole- 

 cules into which the atoms are packed ; and so it became evident that more 

 verified facts must be collected before it will be possible to formulate new 

 generalisations capable of taking their place beside those ordinary ones based 

 upon the conception of simple substances and atoms. But as the shade of the 

 leaves and roots of living plants, together with the relics of a decayed vege- 

 tation, favour the growth of the seedling and serve to promote its luxurious 

 development, in like manner sound generalisations together with the relics 

 of those which have proved to be untenable- promote scientific productivity, 

 and ensure the luxurious growth of science under the influence of rays ema- 

 nating from the centres of scientific energy. Such centres are scientific 

 associations and societies. Before one of the oldest and most powerful of 

 these I am about to take the liberty of passing in review the twenty years' life 

 of a generalisation which is known under' the name of the Periodic Law. It 

 vf&s in March 1869 that I ventured to lay before the then youthful Kussian 

 Chemical Society the ideas upon the same subject which I had expressed in 

 my just written ' Principles of Chemistry.' 



Without entering into details, I will give the conclusion* I then arrived 

 at in the very worfls I used : 



1. The elements, if arranged according 'to their atomic weights, exhibit 

 on evident periodicity of properties. 



4 2. Elements which are similar as regards their chemical properties have 

 atomic weights which are either of nearly the Same value (e.g. platinum, 

 iridium, osmium) or which increase regularly (e.g. potassium, rubidium, 

 caesium). 



' 8. The arrangement of the elements, or of groups of elements, in the 

 order of their atomic weights, corresponds to their so-called valencies as well 

 as, to some extent, to their distinctive chemical properties as is apparent, 

 among other series, in that of lithium, beryllium, barium, carbon, nitrogen, 

 oxygen, and iron. 



4. The elements which are the most widely diffused have small atomic 

 weights. 



*6. The magnitude of the atomic weight determines the character of the 

 dement, just as the magnitude of the molecule determines the character of 

 6 compound. 



4 6. Wo must expect the discovery of many -yet wnknown elements for 

 example, elements analogous to aluminium and silicon, whose atomic weight 

 would be between 65 and 75. 



4 7. The atomic weight of an element may sometimes be* amended by a 

 knowledge of those of the contiguous elements. Thus, the atomic weight of 

 tellurium must lie between 123 and 126, and cannot be 128. 



8, Certain characteristic properties of the elements can be foretold from 

 their atomic weights. 



