INAUGURAL ADDRESS. 15 



US lessons of its own. Moreover, it has been recognised by chemists 

 since the eighteen-sixties that there is a definite relationship among 

 the different kinds of atom which gives to each its own special place 

 in a harmonious scheme, and that this scheme can not be fully 

 understood unless every place be filled. The natural classification 

 of the atoms (or elements) is like one of those children's picture- 

 puzzles which consist of broken parts that have but little meaning 

 till, by patient investigation, their mutual relations are found out, 

 each is fitted in its place, and the picture as a whole becomes visible. 

 Sometimes it happens with these toys that pieces are missing, and 

 gaps must then be left which spoil the picture. So it is with the 

 scheme of the atoms. Mendeleeff put the puzzle together, called 

 attention to the gaps, and, reading the story from the incomplete 

 picture, described some of the missing pieces. How these were 

 found and fitted in, how Mendeleeff's prophetic descriptions were 

 soon after verified by the discoveries of the elements gallium, 

 scandium, and germanium, is an oft-told tale. So is that of the 

 discovery in 1894 of argon by Lord Rayleigh and Sir William 

 Ramsay, and of helium by Ramsay in the following year. But 

 these last discoveries had not been anticipated by Mendeleeff or 

 anyone else. For argon there seemed at first no place in the 

 natural classification of elements : its atom seemed to stand alone, 

 outside the picture that had become so familiar in the preceding 

 thirty years. But helium when discovered resembled argon closely 

 in its curious properties ; and then it became apparent that there 

 was, so to speak, a margin to the picture, hitherto unsuspected, 

 but none the less absolutely essential to its completion ; that argon 

 and helium formed each a piece of this margin, and that other pieces 

 must exist if only they could be found. How Ramsay searched 

 for them and how he found them — the neon piece, the krypton 

 piece, and the xenon piece — these form one of the most fascinating 

 tales of modern science. The discovery of this new group of ele- 

 ments added to our limited list of atoms five new ones, all charac- 

 terised in an unprecedented way by total inability to combine with, 

 other atoms of any kind to produce molecules ; that is to say, these 

 elements proved entirely incapable of playing any part in chemical 

 action. We had long known atoms of different valence, or atom- 

 fixing power, from 1 to 4, or even up to 8, and had for years 

 speculated as to the true nature of this valence which determines 

 the union of atoms in a molecule, and as to the causes of the variation 

 of valence which many atoms indulge in ; but we now learned that 



