148 SCIENCE PROGRESS 



determine the relative weight of the compound atom of water — 

 the product of the union of oxygen and hydrogen— and the 

 number of atoms of each of the elements which unite to form 

 an atom of that compound. As Dalton failed to find a method 

 of solving the first of these problems, he was unable to solve 

 the second. 



The year after the publication of Dalton's New System of 

 Chemical PJiilosopJiy, Gay-Lussac announced, as the result of 

 his experimental investigation of the volumes of gases con- 

 cerned in chemical changes, that there is always such a simple 

 relation between these volumes that all of them can be ex- 

 pressed as small whole multiples of the smallest volume. 

 Dalton interpreted Gay-Lussac's generalisation as equivalent to 

 an assertion that equal volumes of gases contain equal numbers 

 of atoms. To make this statement, Dalton argued, was to 

 declare that all atoms are identical in size and weight. As his 

 theory rested on the assumption that the atoms of different 

 substances differ in size and weight, Dalton refused to accept 

 the generalisation of Gay-Lussac, and declared it to rest on 

 insufficient evidence. 



That he might overcome the difficulty that gravelled Dalton, 

 and also accept the generalisation made by Gay-Lussac, the 

 Italian naturalist Avogadro proposed, in 1811, to distinguish 

 two orders of minute particles — to recognise the molecule as a 

 group of atoms, the atom as a part of the molecule ; and to 

 think of most chemical interactions as separations of molecules 

 into atoms, and rearrangements of the atoms to form new 

 molecules. He made the hypothesis, now generally but errone- 

 ously called Avogadro's /<7zt' (never so called by Cannizzaro), that 

 equal volumes of gaseous elements and compounds, measured 

 at the same temperature and pressure, contain equal numbers 

 of molecules. Avogadro said that if this hypothesis is 

 accepted, the weights of molecules are in the same ratio as the 

 densities of the gaseous elements or compounds which interact. 

 He proposed to take the weight of a molecule of hydrogen 

 as unity. Avogadro determined the molecular weights of 

 several elements and compounds by determining how many 

 times a specified volume of each, in the state of gas, is heavier 

 than an equal volume of hydrogen. Looking back from the 

 position of to-day it may seem to us, that by using the Avoga- 

 drean conception of the molecule as the smallest portion of 



