170 SCIENCE AND THE HUMAN MIND 



weights of the atoms of two elements from a deter- 

 mination of the proportions in which those elements 

 combine in their simplest compound. Thus the com- 

 position of water indicates that the atom of oxygen is 

 eight times as heavy as the atom of hydrogen, and 

 that the constitution of water should be represented 

 as HO, where H stands for one atom of hydrogen 

 and O for one of oxygen. By such methods Dalton 

 assigned atomic weights to some twenty elements 

 (1804). 



The insufficiency of Dalton's conceptions as they 

 stood became apparent when the phenomena of 

 gaseous combination were studied more extensively. 

 Gay-Lussac (1778-1850) showed that gases always 

 combine in volumes that bear simple ratios to each 

 other, and Americo Avogadro, Conte di Quaregna 

 (1776-1856), pointed out that on Dalton's theory it 

 followed that equal volumes of all gases must contain 

 numbers of atoms that bear simple ratios to each 

 other. But it was soon found, both from further 

 study of gaseous combination and from physical con- 

 siderations, that a distinction was necessary between 

 the chemical atom, the smallest part of matter which 

 can enter into combination, and the physical molecule, 

 the smallest particle which can exist in the free state. 

 The simplest method of expression of Avogadro's 

 hypothesis is to suppose that equal volumes of gases 

 contain the same number of molecules. The physical 

 theory developed in mathematical form by Waterston 

 and Joule, that the pressure exerted by a gas is due 

 to the impacts of the molecules, which are in a state 

 of perpetual movement and collision, also leads to 

 this result. 



