ADDRESS. 7 



we have learnt the pace — the fabulous pace — at which the most familiar 

 stars have been for ages approaching to or receding from our planet, 

 without apparently affecting the proportions of the patterns which as far 

 as historical record goes back they have always delineated on the even- 

 ing sky. We have received some information about the elementary atoms 

 themselves. We have learnt that each sort of atom when heated strikes 

 upon the ether a vibration, or set of vibrations, whose rate is all its own ; 

 and that no one atom or combination of atoms in producing its own spec- 

 trum encroaches even to the extent of a single line upon the spectrum 

 that is peculiar to its neighbour. We have learnt that the elements 

 which exist in the stars and specially in the sun are mainly those with 

 which we are familiar upon earth. There are a few lines in excess to 

 which we can give no terrestrial name ; and there are some still more 

 puzzling gaps in our list. It is a great aggravation of the mystery 

 which besets the question of the elements, that among the lines which 

 are absent from the spectrum of the sun, those of nitrogen and oxygen 

 stand first. Oxygen constitutes the largest portion of the solid and liquid 

 substance of our planet, so far as we know it ; and nitrogen is very far the 

 predominant constituent of our atmosphere. If the earth is a detached 

 bit whirled off the mass of the sun, as cosmogonists loAe to tell us, how 

 comes it that in leaving the sun we cleaned him out so completely of his 

 nitrogen and oxygen that not a trace of these gases remains behind to 

 be discovered even by the sensitive vision of the spectroscope ? 



All these things the discovery of spectrum analysis has added to our 

 knowledge ; but it has left us as ignorant as ever as to the nature of 

 the capricious differences which separate the atoms from each other, or 

 the cause to which those differences are due. 



In the last few years the same enigma has been approached from another 

 point of view by Mr. Newlands and Professor Mendeleeff. The periodic 

 law which they have discovered reflects on them all the honour that can be 

 earned by ingenious, laborious, and successful research. The Professor has 

 shown that this perplexing list of elements can be divided into families of 

 about seven, speaking very roughly : that those families all resemble each 

 other in this, that as to weight, volume, heat, and laws of combination 

 the members of each family are ranked among themselves in obedience to 

 the same rule. Each family differs from the others ; but each internally 

 is constructed upon the same plan. It was a strange discovery — strangest 

 of all in its manifest defects. For in the plan of his families there were 

 blanks left ; places not filled up because the properly constituted elements 

 required according to his theory had not been foujid to fill them. For the 

 moment their absence seemed a weakness in the Professor's idea, and 

 gave an arbitrary aspect to his scheme. But the weakness was turned 

 into strength when, to the astonishment of the scientific world, three of 

 the elements which were missing made their appearance in answer to his 

 call. He had described beforehand the qualities they ought to have ; 

 and gallium, germanium, and scandium, when they were discovered 



