42 PRINCIPLES OF CHEMISTRY. 



are designated, and such a representation of a compound substance by 

 symbols is called its formula. Thus, HgO is the formula of the red 

 oxide of mercury, and it tells at once that it is a substance composed 

 of one atom or 200 parts by weight of mercury, and one atom or 16 

 parts by weight of oxygen. 



In the molecule of a compound body there must be at least two 

 atoms, each one of a different element, but there may be in a mole- 

 cule of a compound more than two atoms belonging to two or more 

 elements. 



For instance : The composition of water is H 2 O ; this means, a 

 molecule of water contains 2 atoms of hydrogen and one atom of 

 oxygen. When there is more than one atom of an element in a 

 molecule, the number of these atoms* is designated by placing the 

 figure on the right of the symbol and a little below it, as in H 2 O, 

 whilst 2HO or 2OH would designate 2 molecules of a substance con- 

 taining one atom of hydrogen and one atom of oxygen. 



6. LAWS OF CHEMICAL COMBINATION. 



Law of the constancy of composition. This law, also known 



as the law of definite proportions, was the first ever recognized in 



chemical science ; it was discovered toward the close of the last 



century, and may be stated thus : A definite compound always contains 



r the same elements in the same proportion ; or, in other words, All chemi- 



[ cal compounds are definite in their nature and in their composition. 



To make this law perfectly understood, the difference between a 

 mechanical mixture and a chemical compound must be pointed out. 

 Two powders, for instance sugar and starch, may be mixed together 

 very intimately in a mortar, so that it seems impossible for the eye to 

 discover more than one body. But in looking at this powder with 

 the aid of a microscope, the particles of sugar as well as those of 

 starch may be easily distinguished. The mixture thus produced is a 

 mechanical mixture of molecule clusters. 



QUESTIONS. 41. How does heat act upon the red oxide of mercury? 42. State 

 the difference between mechanical and chemical divisibility. 43. Define the 

 terms element and compound. 44. How many elements and how many com- 

 pound substances are known? 45. What is chemical affinity, and how does it 

 differ from other forces? 46. What is an atom, and how does it differ from a 

 molecule? 47. What is chemistry? 48. Give a definition of atomic weight 

 and of molecular weight. 49. The atom of which element has been selected 

 as the unit for comparison of atomic weights ? 50. Give an explanation of 

 chemical symbols and formulas. 



