CHEMICAL UNITS OF WEIGHT. FORMULAE 77 



The Case of Non-volatile Compounds. The same ele- 

 ments enter also into many compounds which are not volatile. 

 But the unit weights of such elements, determined by the use of 

 volatile compounds, are found (by using multiples, when neces- 

 sary) to express the composition of the involatile compounds also. 



For example, the unit weight for oxygen (16) and that for mer- 

 cury (200.6), both formed by studying volatile compounds, are 

 found correctly to express the composition of mercuric oxide 

 (p. 20), which is not volatile. 



In the cases of some elements no easily vaporizable compound 

 is known, and the unit weight cannot be determined by the present 

 method. In such instances, an entirely different way of obtaining 

 the value of the unit weight is employed (see p. 87). 



( The Law of Combining Weights. The general fact which 



I we have developed in the preceding sections is known as 



, \ the law of combining weights: All the proportions in which the 



/ elements combine with one another may be represented by a set of 



I numbers (one for each element) or by multiples of these numbers 



\by whole numbers. 



Definition of Reacting or Atomic Weight. Many differ- 

 ent values for each element would satisfy the law of combining 

 weights as stated above (see, for example, the values given for 

 oxygen on p. 10). The particular values chosen as units in this 

 chapter, however, fulfil an additional condition which fixes the 

 value in each case, absolutely. The chosen reacting or atomic 

 weight of an element is the smallest weight of the element found 

 in 22.4 liters (at and 760 mm.) of the vapor of any volatile com- 

 pound of that element (p. 74). The amounts of the element in 

 22.4 liters of other compounds are all either the same amount or 

 multiples thereof by a whole number. 



Simplification of Condensed Expressions for Chemical 

 Actions Symbols. Our condensed expressions for chemical 



