FOUNDATION FACTS AND PRINCIPLES 9 



elements in any pure compound is absolutely definite and constant, 

 in strict accordance with the chemical law, and the proportion 

 can be determined with ten times the degree of accuracy required 

 for all practical purposes ; nevertheless, the determination may not 

 be absolutely exact. 



While all atomic weights are essentially referred to hydrogen as 

 unity, the mathematical basis is exactly 16 for the atomic weight 

 of oxygen, because the element oxygen constitutes in quantity 

 one half of the earth's crust (including the air, the ocean, and the 

 solid crust to a depth of ten miles), and forms compounds with 

 nearly all other elements, thus affording closer comparisons than 

 hydrogen. 



The known chemical elements. In the accompanying table is 

 the complete list of 80 known elements. For convenience the more 

 common elements, that every one should know, are given in one 

 group, arid the rare elements, that few people have ever seen, 

 are grouped by themselves. 



The symbols used for the chemical elements are essentially the 

 same in all languages. In a few cases where the modern name varies 

 in different languages, the nations have agreed upon a symbol 

 derived from the Latin name of the element; as, for example, Fe 

 for iron (ferrum in Latin), K for potassium (kalium), and Na for 

 sodium (natrium). 



Some of the atomic weights of the rare elements have not yet 

 been determined with a sufficient degree of accuracy to justify 

 assigning a more specific value than the nearest whole number. 

 Further investigation must determine whether such whole numbers 

 are correct. It is a noteworthy fact that the chemists of the world 

 are agreed that, of these forty more common elements, sixteen 

 have atomic weights that differ from whole numbers by less than 

 .05, and' twelve others differ only by about .1, thus showing 

 twenty-eight of the best-established atomic weights apparently 

 grouped with relation to the unit, with only twelve scattering ; 

 and some of these (as nickel) are doubtful, while others are high 

 atomic weights with consequent possibilities of error, the accepted 

 atomic weights of gold and platinum both having been changed by 

 .5 within recent years. These data and the recognized periodic 

 law, that the properties of the elements are periodic functions of their 



