PRINCIPLES OF CHEMISTRY 109 



The known compounds of nitrogen with oxygen are repre- 

 sented by the following formulae in which each of the capital 

 letters represents one atom of the element : 



Nitrous oxide . . . . . . N 2 



Nitric oxide NO 



Nitrogen trioxide . . . . . . N 2 3 



Nitrogen tetroxide .. .. .. N 2 4 



Nitric peroxide . . . . . . N0 2 



Nitrogen pentoxide . . . . N 2 5 



Obviously if the weight of matter represented by each symbol 

 is known, the relative weights in which the two elements com- 

 bine to form these compounds is also known. The weights 

 attributed to the symbols are called the atomic weights, and 

 what has been learnt about them since Dalton's time will be 

 discussed at length in later pages. 



No sooner is the conception of the atom as the ultimate 

 particle of an element firmly established than it becomes 

 obviously desirable to use some other word to designate the pile 

 of atoms, which, according to the theory, is formed when a 

 chemical compound is produced. Such a word is molecule (dim. 

 of Latin moles, a heap) which, though introduced into science a 

 century ago, has only become during the last fifty years both 

 familiar and endowed with a precise signification. Dalton him- 

 self did not scruple to write of an atom of water, and made no 

 distinction between an atom of an element and an atom of a 

 compound. And in one sense this is justifiable, for what is now 

 called a molecule of water is also an atom (i.e. something indi- 

 visible) inasmuch as if further divided it ceases to be water, and 

 becomes an equal mass of mixed oxygen and hydrogen. 



The word molecule acquired serious importance when it 

 appeared in the title of a paper published in 1811, which, though 

 it attracted comparatively little notice at the time, was at last 

 recognised by the chemical world so long afterwards as 1860. 

 This was the paper by the Italian physicist, Avogadro, in which 

 is enunciated the hypothesis which bears his name, and which 

 is expressed as follows : " Equal volumes of gases, simple or 

 compound, contain under the same conditions of temperature 

 and pressure the same number of molecules." Hence the weights 

 of gaseous molecules are directly proportional to the specific 



