PROGRESS IN NINETEENTH CENTURY 231 



the spectroscope has rendered service of inestimable value, and dis- 

 covery would have been very slow without it. Quite recently, at the 

 very end of the last century and during the few years of the new, re- 

 lations have appeared between the wave-lengths of the spectral lines 

 and the atomic weights of the elements; but the general expression 

 which shall connect them all is yet to be revealed. 



Another discovery in the realm of inorganic chemistry is deserving 

 of mention now on account of its peculiar significance. The atmo- 

 sphere was thought to be well known, and yet in 1895 a new element, 

 argon, was discovered in it. This find was quickly followed by others 

 of like kind, and now five gases previously unknown have been 

 extracted from the air. Each gas is identifiable by its spectrum and 

 its density, and from the latter datum the atomic weight can be 

 inferred. 



Now the interesting fact concerning these atmospheric gases - 

 helium, neon, argon, krypton, and xenon is that they represent 

 matter of a new kind. So far as evidence goes, they are monatomic, 

 absolutely inert, and incapable of union with other elements. Their 

 valence is zero, and when the periodicity of the elements is repre- 

 sented by a vibratory curve, they occupy the points of rest, the 

 nodes. They are matter having physical, but no chemical, properties, 

 and therefore they can be investigated only upon the physical side. 

 This conclusion, perhaps, should be stated provisionally, for it may^ 

 be reversed by future discovery; but of this possibility we have only 

 one suggestion. Helium was first extracted from the mineral uraninite 

 in which it is firmly held, and we cannot say with certainty that it 

 is not chemically combined. Altered or massive uraninite contains 

 little or no helium; the crystallized varieties yield more, and the 

 most brilliant and perfect crystals are the richest of all. The gas 

 may be merely occluded, but the bare chance of combination should 

 not be overlooked. Either supposition is legitimate; but there is 

 still one more possibility, namely, that helium may be generated by 

 the decay of another substance, and not be an original constituent 

 of uraninite at all. Here we touch the mystery of radium a body 

 which challenges our former conceptions of an element, for seemingly 

 it can be decomposed. 



The discovery of radium by Mme. Curie belongs to the nineteenth 

 century, and therefore it falls within the scope of this essay. How 

 it was found, how laboriously the phenomena of radioactivity were 

 observed in order to isolate traces of the new metal, we all know, and 

 the details need no repetition here. At last pure salts of radium were 

 obtained, and the two criteria of spectrum and atomic weight were 

 satisfied. Radium is clearly a metal of the barium group, it fills a 

 definite place in the periodic table, its claims to elementary rank are 

 on a level with those of other elements, and yet it exhibits an apparent 



