LAWS OF CHEMICAL COMBINATION. 41 



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

 a molecule of a compound more than two atoms belonging to 

 two or more elements. 



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

 a molecule of water contains 2 atoms of hydrogen and 1 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 tigure on the right of the symbol and a little below it, as in 

 H 2 0, whilst 2HO or 2(H(J) would designate 2 molecules of a 

 substance containing one atom of hydrogen and one atom of 

 oxygen . 



QUESTIONS. 



41. How does heat act upon 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 compound 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 ibeen selected as the unit for 

 comparison of atomic weights ? 



50. Give an explanation of chemical symbols and formulas. 



6. LAWS OF CHEMICAL COMBINATION. 



Law of the constancy of composition. This law was the first 

 ever recognized in chemical science; it was discovered towards 

 the close of the last century, and may be stated thus : A definite 

 compound always contains the same elements in the same proportion ; 

 or, in other words : All chemical 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 by the aid of a microscope, the particles 



