THE PERIODIC LAW 1 



THE name "Periodic Law" is given to the generally ac- 

 cepted embodiment of the relations existing between the 

 various properties of the chemical elements, so far as they 

 can be compared with one another. It may be stated as fol- 

 lows : // the elements are arranged in the order of their atomic 

 weights, each of their properties varies as a periodic function 

 of the atomic weight. 



Ever since the work of Richter, Proust, and Dalton had 

 established the idea of fixed numerical values attaching to 

 the ingredients of compounds (an idea which was deduced by 

 Dalton from the hypothetical existence of individual atoms, 

 identical in size, mass and other properties for any one ele- 

 ment), chemists sought to deduce a closer relationship be- 

 tween the various elements from a comparison of the masses 

 of their respective atoms. The first attempt was that made by 

 Dr. Prout in 1815 to prove that all the atomic weights were 

 even multiples of the atomic weight of hydrogen, and that 

 the latter was the only primitive element, from which the 

 others were derived by processes of condensation. It was 

 soon found that very few elements possessed atomic weights 

 that could be expressed by integers, when the atomic weight 

 of hydrogen was set at unity, and Prout's law was gradu- 

 ally modified to state that one half the atomic weight of hy- 

 drogen, then that one-quarter, should be taken as the real 

 standard. Refinements of investigation have since es- 

 tablished the relative atomic weights to the second place 

 of decimals, and it can now be asserted that the number 



1 Reprinted from the New International Encyclopaedia (Dodd, Mead & Co., New 

 York, 1912, 15, 593-597), by permission of the Publishers. 



