38 THE POPULAR SCIENCE MONTHLY. 



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 spec- 

 trum of the sun, those of nitrogen and oxygen stand first. Oxy- 

 gen constitutes the largest portion of the solid and liquid sub- 

 stance 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 oif the mass of the sun, as cosmogonists 

 love 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 sensi- 

 tive vision of the spectroscope ? 



All these things the discovery of the 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 Prof. MendeMeff. The periodic 

 law which he has discovered reflects on him all the honor that 

 can be earned by ingenious, laborious, and successful research. 

 He 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 found to 

 fill them. For a 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, ger- 

 manium, and scandium, when they were discovered shortly after 

 the publication of his theory, were found to be duly clothed with 

 the qualities he required in each. This remarkable confirmation 

 has left Mendel^eff's periodic law in an imassailable position. 

 But it has rather thickened than dissipated the mystery which 

 hangs over the elements. The discovery of these co-ordinate 

 families dimly points to some identical origin, without suggesting 

 the method of their genesis or the nature of their common par- 



