34 CHEMISTRY. 



2 atoms of hydrogen, or II 2 , weigh 2 



1 atom of sulphur, or S, weighs 32 



4 atoms of oxygen, or O 4 , weigh 64 



1 molecule of sulphuric acid, or H S SO 4 , weighs 98 



You notice that we have drawn a line under the weights of 

 the atoms and added them together, obtaining the weight 

 of a molecule of sulphuric acid. Atoms, you remember, 

 unite to form molecules, and here you see that the sum of 

 the atomic weights of the elements composing the mole- 

 cule of a body gives the weight of a molecule of that body. 

 This is called the Law of Molecular Weights. 



Take another example : Common limestone, you will learn 

 farther on, is a compound of calcium, carbon, and oxygen, 

 in the proportions represented by the formula CaCO 3 : 



1 atom of calcium, or Ca, weighs 40 



1 atom of carbon, or C, weighs 12 



3 atoms of oxygen, or O 3 , weigh 48 



1 molecule of calcium carbonate, or CaC0 3 , weighs 100 



The molecular weight of crystallized gypsum can be cal- 

 culated in like manner, the weight of two molecules of 

 water (2H 2 O = 36) being added to that of one molecule, of 

 gypsum. Use the Table on page 13 to make this calcula- 

 tion. These examples also make the Law of Definite Pro- 

 portions much clearer. 



31. Other Laws of Chemical Combination. In explaining 

 the meaning of symbols, we have somewhat anticipated the 

 other laws of chemical combination. The Law of Multiple 

 Proportions may be thus stated: When one body com- 

 bines with another in several proportions, the higher propor- 

 tions are multiples of the first or lowest. This results from 

 the fact that one, two, three, or more atoms of one element 

 often unite with a single atom of some other element, form- 

 ing three or more bodies having very different properties. 



