THE MOLECULAR COMPOSITION OF MATTER 85 



all atomic weights in terms of oxygen taken as 16 rather 

 than hydrogen as 1. The atomic ' weight of hydrogen 

 as expressed on this scale is 1.008. 



If we wish to deal always with the same number of 

 molecules we may do so, then, by taking of each sub- 

 stance a number of grams equal to its molecular weight. 

 Thus 32 grams of oxygen and 2.016 grams of hydrogen 

 contain the same number of molecules. Such an 

 amount of any substance is known as "1 gram-mole- 

 cule/' or by abbreviation as "1 gm.-mole," or more 

 simply as "1 mole." 



The combination of atoms in groups, which we 

 symbolized in terms of A and B earlier in this chapter, 

 represent of course but a few of the combinations 

 which we can imagine. Why do some of these com- 

 binations occur hi chemical reactions and not others? 

 Why, for example, does one hydrogen atom combine 

 with one chlorine atom to form hydrochloric acid, HC1, 

 but two hydrogen atoms combine with one of oxygen 

 to form water, H 2 O? Why do we not obtain a com- 

 pound like HC1 2 or H 2 C1? What is the cause of such 

 combinations ? 



i Speculations as to the cause have been made since 

 the time of the early Greek philosophers. Hippoc- 

 rates assumed that "like draws to like" and that 

 substances which combined had something in common. 

 The early Greek atomists personified the atoms, at- 

 tributing to them loves and hates. The medieval 

 alchemists were more picturesque in their ideas. They 

 assumed that the combinations were due to differences 

 in the forms of the particles of substances. They 

 pictured acids as composed of sharp particles like 



