526 THE POPULAR SCIENCE MONTHLY. 



after a laborious search, found that it belonged to yttrium. Subse- 

 quent studies showed this modification of spectrum analysis to exceed 

 in delicacy all known tests for the rarer earths ; yttrium can be de- 

 tected when present in one millionth part. Within a twelvemonth, 

 Crookes has made known the application of radiant matter spec- 

 troscopy to samarium ; the delicacy of this test surpasses that for 

 yttrium, and the anomalous behavior of the mixed earths yields phe- 

 nomena "without precedent." 



When Dalton, the Manchester schoolmaster, added to the atomic 

 theory of the Greeks the laws of definite and of multiple proportions, 

 he transformed an "interesting intellectual plaything" into an exact 

 scientific theory capable of experimental demonstration. The impor- 

 tance of ascertaining the atomic weights of the elements with the ut- 

 most accuracy has stimulated chemists to apply to the problem their 

 best endeavors ; and as the methods of analysis become more refined, 

 the determinations are again and again repeated, every ascertainable 

 and imaginable source of error being carefully eliminated. Besides the 

 experimental repetitions, the figures obtained by various observers 

 have recently been submitted to careful recalculations by Clarke in 

 this country, and soon after by Lothar Meyer and Seubert, in Ger- 

 many. Their labors give chemists the latest and most reliable con- 

 stants. 



For many years chemists have dimly perceived the probable corre- 

 lation of the properties of the elementary bodies and their atomic 

 weights. Dumas pointed this out for certain marked groups, New- 

 lands emphasized it ; but it remained for a Russian chemist, Mende- 

 lejeff, to establish, in 1869, a law of great importance. Mendelejeff 

 showed that if the elements are grouped in the order of their atomic 

 weights, it will be found that nearly the same properties recur peri- 

 odically throughout the entire series. This so-called Periodic Law is 

 more concisely stated thus : The properties of the elements are peri- 

 odic functions of their atomic weights. The accuracy of the deduc- 

 tions based on this law is strikingly shown by the fact that Mendele- 

 jeff, finding an unfilled blank in the periodic system, boldly announced 

 the general and special properties of the element awaiting discovery ; 

 six years later, Lecoq de Boisbaudran discovered gallium, an element 

 which proved to have properties almost identical with those of the 

 hypothetical eha-aluminiurn described by Mendelejeff. And in 1879 

 the accuracy of Mendelejeff's prophecy was further confirmed by Nil- 

 son's discovery of scandium, the counterpart of the hypothetical eka- 

 bor. JEka-silicon, though yet to be discovered, may almost be re- 

 garded as a known element, so fully have its properties been predicted. 



The correlation between atomic weights and physical properties is 

 being extended, and now embraces the fusibility, boiling-points, gen- 

 eral affinities, color, occurrence in nature, physiological functions, and 

 many other factors. Dr. Carnelley, who has been active in develop- 



