THE REIGN OF LAW 165 



or three bulks of one to one of another." When 

 Dalton had investigated the relative weights with 

 which elements combine, he had found no simple 

 arithmetical relationship between atomic weight and 

 atomic weight. When two or more compounds of the 

 same elements are formed, Dalton found, however, 

 as we have seen, that the proportion of the element 

 added to form the second or third compound is a 

 multiple by weight of the first quantity. Gay-Lussac 

 now showed that gases, " in whatever proportions 

 they may combine, always give rise to compounds 

 whose elements by volume are multiples of each 

 other." 



In 1811 Avogadro, in an essay on the relative 

 masses of atoms, succeeded in further confirming 

 Dalton's theory and in explaining the atomic basis of 

 Gay-Lussac's discovery of simple volume relations in 

 the formation of chemical compounds. According to 

 the Italian scientist the number of molecules in all 

 gases is always the same for equal volumes, or al- 

 ways proportional to the volumes, it being taken for 

 granted that the temperature and pressure are the 

 same for each gas. Dalton had supposed that water 

 is formed by the union of hydrogen and oxygen, 

 atom for atom. Gay-Lussac found that two volumes 

 of hydrogen combined with one volume of oxygen to 

 produce two volumes of water vapor. According to 

 Avogadro the water vapor contains twice as many 

 atoms of hydrogen as of oxygen. One volume of 

 hydrogen has the same number of molecules as one 

 volume of oxygen. When the two volumes combine 

 with one, the combination does not take place, as 

 Dalton had supposed, atom for atom, but each half- 



