i64 POPULAR SCIENCE MONTHLY. 



which lies between the extremes, the type element, silicon, foreshadows 

 the members of its two series to an approximately equal extent. 



As the tables which graphically portray the Periodic Law stand 

 to-day, there is much which still remains to be cleared up. At the very 

 outset we are met by the fact that we cannot tell in which group as 

 familiar an element as hydrogen ought to be placed. It generally re- 

 ceives the first place in group one, but to some extent at least this 

 position is based upon a misapprehension. Some years ago Pictet, of 

 Geneva, was engaged in that work on the condensation and liquefaction 

 of gases, which has rendered his name famous. On compressing hydro- 

 gen at a very low temperature, he obtained, on suddenly reducing the 

 pressure, some heavy, steel-blue drops, much resembling mercury. This 

 was erroneously supposed to be hydrogen in the liquid form. As a 

 result, it seemed only natural to classify hydrogen with the metallic 

 elements of the first group. Not only have later investigations shown 

 that these drops were not liquid hydrogen, but quite recently Dewar 

 has actually obtained this substance, which proves to be a colorless, 

 limpid liquid, with the extraordinarily low specific gravity of about 0.07. 

 As far then as physical properties go, there is no justification in classify- 

 ing hydrogen with the metals of group one. Chemically, however, 

 hydrogen is like the metals, electro-positive, though very weakly so, 

 and it is possible that its position in the first group is less awkward than 

 would be any other. 



Of the other elements in the table with atomic weight below 100, 

 all seem fairly well placed, though we have not as much knowledge of 

 scandium as we could wish, and there is a difficulty that we shall soon 

 notice in the eighth group. The first apparent blank space in the table 

 is for an element with atomic weight of about 100. Such an element 

 would be known as eka-manganese, and would possess properties which 

 would to a considerable extent resemble those of manganese, but per- 

 haps more closely those of ruthenium. Beyond this in the table we find 

 many gaps, partly from the inadequacy of our chemical knowledge and 

 partly from the likelihood that there exist rare elements which have 

 not yet been discovered. Such elements probably occur in extremely 

 small quantities, and may, for many years, perhaps forever, elude 

 chemists. It seems improbable that there are undiscovered elements 

 which exist in more than very small quantities; this is the testimony, 

 not only of the chemical laboratory, but also of the spectroscope, that in- 

 strument which reveals to us the composition, not merely of substances 

 in the laboratory, but also that of the sun and of the distant stars. 

 From barium to tantalum few elements are known to which a definite 

 place can be assigned, but here there is an indefinitely large number 

 of what Crookes has called meta-clements, the rare earths. Their rela- 



