THE STORY OF NINETEENTH-CENTURY SCIENCE 



observations of the French chemist as valid. Yet the 

 observations of Gay-Lussac were correct, as countless 

 chemists since then have demonstrated anew, and his 

 theory of combination by volumes became one of the 

 foundation-stones of the atomic theory, despite the op- 

 position of the author of that theory. 



The true explanation of Gay-Lussac's law of combina- 

 tion by volumes was thought out almost immediately by 

 an Italian savant, Amadeo Avogadro, and expressed in 

 terms of the atomic theory. The fact must be, said 

 Avogadro, that under similar physical conditions every 

 form of gas contains exactly the same number of ulti- 

 mate particles in a given volume. Each of these ulti- 

 mate physical particles may be composed of two or more 

 atoms (as in the case of water vapor), but such a com- 

 pound atom conducts itself as if it were a simple and 

 indivisible atom, as regards the amount of space that sep- 

 arates it from its fellows under given conditions of press- 

 ure and temperature. The compound 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 gen- 

 eration. 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. 



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