i86 GREAT MEN OF SCIENCE 



and ammonia, carbon monoxide and oxygen, and many 

 others, and in every case he found no less simple relation- 

 ships by volume. He also calculated in part the rela- 

 tionships by volume, which would appear in the gaseous 

 state, from the known relationship by weight and the speci- 

 fic gravities of the gases or vapours. He is thereupon rightly 

 able to announce 'that two different kinds of gas, when they 

 act chemically upon one another, always combine in the most 

 simple relationship by volume,' and also that 'the volume of 

 the gaseous products or combination is likewise always a 

 simple proportionate number.' 



This fact, which powerfully supported Dalton's atomic 

 ideas, was one of the strongest bases for the further de- 

 velopment of all knowledge of atoms and molecules, and 

 hence of all investigation of the structure of matter, par- 

 ticularly in the gaseous state. It was first of all evident that, 

 in the simplest case, an equal number of atoms or possibly 

 molecules of different gases would be present in the same 

 volume. In this connection, the notions of atom and mole- 

 cule were not at that time distinctly separated. Amadeo 

 Avogadro (i 776-1 856), in Turin, was the first who, already 

 in 181 1, decided to assume the existence of special molecules, 

 even in the case of the elementary gases, not consisting of 

 one, but, at least in the case of the ordinary gases then 

 known, of two similar atoms combined together, whereas 

 the molecules of chemical compounds consist of unlike atoms 

 combined together. When the conception of molecules had 

 thus been defined, it was possible to assume the existence in 

 all gases of equal numbers of molecules in equal volumes, 

 by which agreement was found to obtain with all experi- 

 mental data of chemistry known at the time. All this neces- 

 sarily remained for a long time mere hypothesis (supposi- 

 tion), and it was, quite justifiably, only accepted with great 

 diffidence. But all subsequent experience only served to 

 confirm it; though it was only put beyond doubt almost fifty 



