So AN INTRODUCTION TO SCIENCE 



example, if we have reason to believe that hydrochloric acid 

 gas is formed by the union of hydrogen atoms and chlorine 

 atoms in equal numbers, its formula may be expressed as 

 HC1 ; and if, when it is analysed, this gas is found to contain 

 by weight 35^ times as much chlorine as hydrogen, the 

 atom of chlorine weighs 35^ in the hydrogen scale. 



We may digress for a moment to explain how it comes to 

 be possible to ascertain whether the same number of atoms 

 of two elements are in combination, or unequal numbers. 

 Cavendish found that when he filled a globe with a mixture 

 of pure hydrogen and pure oxygen in the proportion of 2 

 cubic inches of hydrogen for every i cubic inch of oxygen, 

 and exploded the mixture, nothing remained but water. 

 Except for the water-vapour the globe was empty. It could 

 be refilled with the same mixture over and over again, and 

 yet after several explosions nothing remained in it but 

 water. Gay-Lussac and Humboldt (1805) repeated this ex- 

 periment, not only with hydrogen and oxygen, but with hy- 

 drogen and chlorine, and various other gases which combine 

 to form gaseous products, and they found that the volumes 

 of any two gases which must be used if a compound is to be 

 formed and no remainder left over, bear such simple 

 numerical relations one to another, as i : i, 2 : i, 3 : 4, 

 etc. This discovery, considered in its bearing upon Dai- 

 ton's observation that the proportions by weight in which 

 any given element enters into the formation of several dis- 

 tinct compounds, bear very simple numerical relations one 

 to another, led to the formulation by Berzelius of Uie 

 theorem that equal volumes of gases contain equal numbers 

 of atoms. Berzelius' generalisation was fallacious, because 

 he did not know that even in the gaseous elements the 

 atoms are not isolated but combined into molecules. When 

 Avogadro substituted the word molecule for atom, and said 

 that "equal volumes of all gases contain equal numbers of 

 molecules," the theorem assumed an expression which is 

 subject to no dispute. No matter what gas is put into a 



