THE PRINCIPLES OF ORGANIC CHEMISTRY 45 



This somewhat lengthy description of the methods and 

 arguments involved in arriving at the molecular formula for 

 an organic compound has been entered into, because it appears 

 of fundamental importance that the real meaning of a mole- 

 cular formula should be properly understood, as all other 

 developments in regard to the molecular structure of com- 

 pounds depend upon this. 



A molecular formula tells us how many atoms of each con- 

 stituent element are present in the molecule of the compound. 

 It tells us nothing, however, as to the way in which these 

 atoms may be combined within a molecule. The extra- 

 ordinary advances which modern chemistry has made in the 

 study of the arrangement of the atoms within the molecule, 

 a study which must necessarily precede a systematic attempt 

 to build up these molecules from their constituent elements, 

 naturally had to begin with the study of the simplest com- 

 pounds. Supposing we take the following simple compounds 

 of carbon, whose molecular weight and molecular formulae are 

 easily ascertained by the methods already indicated : — 



Carbon monoxide, CO ; 

 Carbon dioxide, CO3 ; 

 Methane, CHi ; 

 Chloroform, CHCI3 ; 

 Hydrocyanic acid, HCN ; 



we see that one atom of carbon is able to combine with one or 

 two atoms of oxygen ; with four atoms of hydrogen ; or with 

 one atom of hydrogen and one atom of nitrogen. We also 

 know that one atom of oxygen combines with two atoms of 

 hydrogen to form water, H3O ; that one atom of hydrogen 

 combines with one atom of chlorine to form hydrochloric acid 

 gas, HCl ; further that one atom of nitrogen combines with 

 three atoms of hydrogen to form ammonia, NH3. 



If we study the formulae of the five compounds of carbon 

 given in the above list in the light of these facts, we shall see 



