352 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



enter as a whole into combination with other elements and 

 are not capable of being further. subdivided. "Diatomic" 

 molecules, although abundant amongst the commoner gases, 

 are by no means universal ; in a complete table of volatile 

 elements the diatomic molecules are seen to be less abun- 

 dant than the monatomic : 



Monatomic Na, K, Zn, Cd, Hg, He, Ne, Ar, Xe, Kr, I 

 Diatomic H 2 , N 2 , O 2 , F 2 , C1 2 , Br 2 , I 2 , S 2 



TriatomiC) 6<r. O 3 , P 4 , As 4 , S s . 



Formulae of volatile compounds deduced by Cannizzaro. 



tlaving established a method of determining the atomic 

 weights of their constituents, Cannizzaro was able to assign 

 formulae to a large number of volatile compounds. 



Thus in the case of ether, the molecular weight, 74, 

 deduced from its vapour density, was shown by the analy- 

 tical data to contain 



Carbon 48, Hydrogen 10, Oxygen 16 parts. 



Assuming the weights of the atoms to be 

 Carbon 12, Hydrogen i, Oxygen 16 



it was clear that the numbers of atoms in the molecule were 



Carbon i- = 4 Hydrogen = 10 Oxygen = i 



12 I IO 



and that the formula of the substance was 

 C 4 H 10 0. 



Formulae deduced in this way, being based directly upon 

 Avogadro's hypothesis, were entirely independent of the 

 personal bias which had produced so much confusion 

 .during the preceding half-century; it is therefore not sur- 

 prising that they soon met with universal acceptance. 



The complexity of gaseous elements confirmed by 

 measurements of their heat capacity. The conclusions of 



