CHEMISTRY SINCE TIME OF DALTON 



bination by volumes was thought out almost immedi- 

 ately by an Italian savant, Amadeo Avogadro, and ex- 

 pressed in terms of the atomic theory. The fact must 

 be, said Avogadro, that under similar physical condi- 

 tions every form of gas contains exactly the same 

 number of ultimate particles in a given volume. Each 

 of these ultimate physical particles may be composed 

 of two or more atoms (as in the case of water vapor), 

 but such a compound atom conducts itself as if it were 

 a simple and indivisible atom, as regards the amount 

 of space that separates it from its fellows under given 

 conditions of pressure and temperature. The com- 

 pound atom, composed of two or more elementary 

 atoms, Avogadro proposed to distinguish, for purposes 

 of convenience, by the name molecule. It is to the 

 molecule, considered as the unit of physical structure, 

 that Avogadro's law applies. 



This vastly important distinction between atoms and 

 molecules, implied in the law just expressed, was pub- 

 lished in 1811. Four years later, the famous French 

 physicist Ampere outlined a similar theory, and utilized 

 the law in his mathematical calculations. And with 

 that the law of Avogadro dropped out of sight for a full 

 generation. Little suspecting that it was the very key 

 to the inner mysteries of the atoms for which they were 

 seeking, the chemists of the time cast it aside, and let it 

 fade from the memory of their science. 



This, however, was not strange, for of course the law 

 of Avogadro is based on the atomic theory, and in 181 1 

 the atomic theory was itself still being weighed in the 

 balance. The law of multiple proportions found general 

 acceptance as an empirical fact ; but many of the lead- 



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