36 CHEMISTRY. 



Referring again to the Table of Atomic Weights, we 

 find that 32 parts by weight of sulphur combine with 56 

 parts by weight of iron and with 16 parts by weight of 

 oxygen, consequently 56 parts by weight of iron combine 

 with 16 parts by weight of oxygen. Or, using less wordy 

 language, since one atom of sulphur (32) combines with one 

 atom of iron (56) and with one atom of oxygen (16), these 

 bodies, iron and oxygen, combine with each other atom, 

 to atom (56 and 16). Actually the relation is not quite so 

 simple, for oxygen and sulphur combine usually in the 

 proportion of two atoms of the former to one of the latter, 

 and this necessitates the use of the word " multiples " in 

 the law as just stated. 



We might in a similar way go through the whole list of 

 elements, showing that their atomic weights express their 

 value in satisfying the demands of each other in their com- 

 binations. 



32. Summary. We will sum up in four propositions the 

 facts which we have developed in this chapter in regard to 

 the combinations of substances. l.When substances com- 

 bine with each other, it is always in certain fixed and in- 

 variable proportions. In other words, every compound al- 

 ways has precisely the same composition. This is called 

 the law of definite proportions. 2. When two substances 

 unite in more proportions than one, these proportions bear 

 a simple arithmetical relation to each other. This is called 

 the law of multiple proportions. Commonly, as you have 

 seen, one of the substances remains the same, while the 

 other is in different proportions, usually as 1,2, 3, etc. 

 Sometimes the relation is as 1 J, 2-J-, 3J, etc.; but this we shall 

 speak of in another place. 3. When several substances, 

 B, C, D, etc., unite with a substance, A, the proportions in 

 which they unite with it are expressed by numbers, which 

 represent the proportions in which they unite with each 



