lO Clarke, The Atomic Theory. 



been influenced by Richter rather than by Dalton, but 

 that bias was only temporary. For more than thirty years 

 Berzelius continued these labours, inventing symbols, 

 establishing formula?, and determining atomic weights. 

 He, above all other men, made the atomic theory 

 applicable to general use, a universal tool suited to 

 practical purposes. Turner, Penny, Erdmann and others 

 did noble work of the same order, but Berzelius over- 

 shadowed them all. Throughout his long career he was 

 almost the dictator of chemistry. 



It was on the physical side, however, that the theory 

 of Dalton was most profoundly modified. First came the 

 researches of Gay Lussac, who in 1808 showed that 

 combination between gases always took place in simple 

 relations by volume, and also that all gaseous densities 

 were proportional either to the combining weights of the 

 several substances, or to rational multiples thereof In 

 181 1 Avogadro generalised the new evidence, and brought 

 forward the great law which is now known by his name. 

 Equal volumes of gases, under like conditions of tempera- 

 ture and pressure, contain equal numbers of molecules. 

 Mass and volume were thus covered by one simple 

 expression, and both were connected with the weights of 

 the fundamental atoms. Avogadro, moreover,distinguished 

 clearly between atoms and molecules, a distinction which is 

 of profound importance to chemistry, although it is not 

 always properly appreciated by students of physics. The 

 molecule of to-day, which is usually, but not always, a cluster 

 of atoms, is identical with the atom of the pre-Daltonian 

 philosophers ; while the chemical unit represents a new 

 order of divisibility which the Ancients could never 

 have imagined. A molecule of water was easily conceived 

 by them, but its decomposition into smaller and simpler 

 particles of ox}'gen and hydrogen, the chemical atoms, 



