ONE HUNDRED YEARS OF CHEMISTRY 309 



Atomic Numbers. As the result of a spectroscopic 

 study of the wave-lengths or frequencies of the X-rays 

 produced when cathode rays strike upon anti-cathodes 

 composed of different elements, Moseley in 1914 discov- 

 ered that whole numbers in a simple series can be 

 attributed to the atoms. These atomic numbers are: 1 

 for hydrogen, 2 for helium, 3 for lithium, 4 for beryllium, 

 and so on, in the order in which the elements occur in 

 Mendeleeff: 's periodic table, and in the cases of argon and 

 potassium, cobalt and nickel, and tellurium and iodine, 

 they follow the correct chemical order, while the atomic 

 weights do not. They appear to indicate, therefore, an 

 even more fundamental relation between the atoms than 

 that shown by the atomic weights. 



These numbers are now available for every element 

 up to lead, and they are particularly interesting in indi- 

 cating, on account of missing numbers, the existence of 

 two undiscovered elements in the manganese group, and 

 two more among the rare-earth metals, in confirmation 

 of the vacant places below lead in Mendeleeff 's table. 



The Isolation of Elements. In the year 1818 about 

 53 elements were recognized, and since that time about 

 30 more have been discovered, but the elements already 

 known comprised the more common ones, and nearly all 

 of those which have been commercially important. A 

 few of them, including beryllium, aluminium, silicon, 

 magnesium, and fluorine, were then known only in their 

 compounds, as they had not yet been isolated in the free 

 condition. 



Berzelius in 1823 prepared silicon, a non-metallic 

 element resembling carbon in many respects. This 

 element has recently been prepared on a rather large 

 scale in electric furnaces at Niagara Falls, and has been 

 used for certain purposes in the form of castings. 



Wohler created much sensation in 1827 by isolating 

 aluminium and finding it to be a very light, strong and 

 malleable metal, stable in the air, and of a silver-white 

 color. For a long time this metal was a comparative 

 rarity, being prepared by the reduction of aluminium 

 chloride with metallic sodium; but about 25 years ago 

 Hall, an American, devised a method of preparing it by 



