THE PERIODIC LAW 89 



compounds they form in such wealth and variety, 

 their spectra, and the relative weights of their atoms, 

 down to the merest minutiae and with an accuracy 

 unsurpassed in quantitative science. 



He discovered the most curious family re- 

 semblances between them, some being so similar 

 in their whole character and so regular even in their 

 differences that no discipline of the imagination 

 could entirely suppress the private question, "What 

 are they ? " even though the memory of those early 

 heresies about transmutation and the unity of matter 

 made it bad form to romance about them. Lastly, 

 he made, when he put out the elements in the order 

 of the relative weights of their atoms beginning 

 with hydrogen, the lightest atom, and ending with 

 uranium, the heaviest a sweeping generalisation 

 about them known as the Periodic Law. Essentially 

 this is that nearly the whole of the properties of 

 the elements are periodically recurring functions of 

 their atomic weights. The tenth element in the 

 list has a close family resemblance to the second, 

 the eleventh to the third, the twelfth to the fourth, 

 and so on to the seventeenth which is like the 

 ninth. The eighteenth is like the second and 

 tenth, the nineteenth like the third and eleventh. 

 Hydrogen, the first element, stands alone and has 

 no analogues. After the twenty-second element, 

 titanium, a change in the nature of the periodicity 

 occurs, which becomes more complex. Another very 

 abrupt change occurs at the fifty-sixth element, 

 barium, when the rare-earth elements commence. 

 These, the next thirteen or fourteen elements, all 

 resemble one another with extreme closeness, in 

 direct contradiction to what occurs with the elements 

 both before and after them in the list. At the 

 seventy-third element, tantalum, the law departed 

 from at the fifty-sixth element is reverted to again 



