THE MOLECULAR COMPOSITION OF MATTER 81 



it would be represented by AB 2 , where the subscript 

 represents the number of atoms, of the kind indicated 

 by the letter, which enter into the formation of the 

 molecule. Other possible combinations are A 2 B, A 2 B 2 , 

 A 2 B 3 , AsB, AsB 2 , and so on. Whether or not any 

 particular one of the compound molecules which we 

 have represented can be formed will depend upon the 

 mechanism by which the atoms of a molecule are held 

 together. This we shall discuss in the next chapter. 



Let us now see what experiment Dalton performed to 

 test such an assumption as to molecular composition. 

 In 1802 he discovered that if he mixed 100 parts of 

 common air with 36 parts of nitric oxide he could obtain 

 a combination of all the oxygen of the air with this 

 oxide, leaving a residue of 79 parts of atmospheric 

 nitrogen. Such a combination l occurred if the mixture 

 was made in a narrow vessel or tube. On the other 

 hand, if the combination took place hi a large vessel 

 over water, in which case it would be very rapid, a 

 different compound 2 was obtained. In this case he 

 could mix 100 parts of air with 72 parts of nitric oxide 

 and all the oxygen would enter into combination, as 

 would be evidenced by the reduction of the air to 79 

 parts as before. Evidently the same amount of oxygen 

 could enter into combination with a definite amount 

 of nitric oxide or with twice that amount. But Dalton 



1 Nitric oxide is NO. This compound is nitric peroxide, or in 

 symbols NO 2 . Thus 2NO+O 2 =2NO 2 . 



2 The compound thus formed was nitrous anhydride or N 2 O 3 . 

 Four molecules of NO combine with one of oxygen, containing two 

 atoms, as may be expressed in the equation 4NO+O 2 =2N 2 O S . 

 Other compounds which may be formed are N a O or nitrous oxide, 

 and N 2 O 6 or nitric anhydride. 



Q 



