xv THE MOLECULAR THEORY 343 



In the case of each of the gaseous elements (hydrogen, 

 oxygen, nitrogen and chlorine) discussed by Avogadro in 

 this section of his memoir, the molecule was found to be 

 divisible into two parts, but no further division was observed. 

 The indivisible atom was therefore probably the half-molecule 

 in the case of each of these gases. It should be observed, 

 that the facts prove no more than that the molecule can be 

 divided into some even number of atoms. In a later section 

 further reasons will be given in support of the belief that in 

 each of these cases not more than two atoms are contained 

 in the molecules ; but other cases are known in which the 

 molecules of a gas can be divided into 3, 4, 6 or 8 atoms. 



A vogadro's hypothesis neglected for nearly fifty years. 

 It was unfortunate that Avogadro, having found a trustworthy 

 method of determining the relative weights of gaseous 

 molecules did not limit his deductions to those volatile 

 compounds and elements to which alone his hypothesis 

 could be applied. In attempting to extend his views to 

 non-volatile compounds, such as the oxides and salts of the 

 metals, Avogadro was compelled, like Dalton, to fall back 

 upon mere guess-work. His speculations failed to secure 

 the support of his contemporaries and may well have pre- 

 judiced them against the accurate conclusions which he 

 reached in the case of gaseous elements. But, whatever 

 'the cause, the fact remains that nearly half a century elapsed 

 before A vogadro's views, re-stated and applied, in 1858, by 

 Stanislao Cannizzaro (18261910), Professor of Chemistry 

 at Genoa (A.C.R. XVIIL), finally secured the universal 

 recognition that they deserved. 



D. DETERMINATION OF MOLECULAR AND ATOMIC 

 WEIGHTS. 



Cannizzaro (1858) chooses the half -molecule of hydrogen 

 as a standard of molecular and atomic weights. In 



