518 



SCIENCE. 



[N. S. Vol. XVIII. Xu. 400. 



childhood of the doctrine, before its vigor- 

 ous growth had thoroughly begun. 



The development of the atomic theory 

 follovfed two distinct lines, the one chem- 

 ical, the other physical, in direction. On 

 the chemical side the leader was Ber- 

 zelius, who began in 1811 the publication 

 of his colossal researches upon definite pro- 

 portions. At first he seems to have been 

 influenced by Richter rather than by Dal- 

 ton, but that bias was only temporary. 

 For more than thirty years Berzelius con- 

 tinued these labors, inventing symbols, es- 

 tablishing formulae 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 overshadowed 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 com- 

 bination 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 sev- 

 eral substances, or to rational multiples 

 thereof. In 1811 Avogadro generalized the 

 new evidence, and brought forward the 

 great law which is now known by his name. 

 Equal volumes of gases, under like con- 

 ditions of temperature and pressure, con- 

 tain 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 chem- 

 istry, although it is not always properly ap- 

 preciated 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 phi- 

 losophers; while the chemical unit repre- 

 sents 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 oxygen and hy- 

 drogen, the chemical atoms, was far beyond 

 the range of their knowledge. That the 

 distinction is not always borne in mind by 

 physicists is illustrated by the fact that in 

 Clerk Maxwell's article 'Atom,' in the 

 'Encyclopaedia Britannica,' Dalton is not 

 even mentioned, and that the phenomena 

 there selected for discussion are molecular 

 only. Maxwell was surely not ignorant of 

 the diflEerence between atoms and molecules, 

 but his knowledge had not reached the 

 point of complete realization. His thought 

 was of molecules, and so Maxwell uncon- 

 sciously neglected the real subject of his 

 chapter, the atom. Of late years many es- 

 says upon the atomic theory have been writ- 

 ten from the physical side, and few of 

 them have been free from this particular 

 ambiguity. At first, a similar error was 

 committed by chemists, who paid small at- 

 tention to Avogadro 's law, and so the latter 

 failed to exert much influence upon chem- 

 ical thought until more than forty years 

 after its promulgation. The relation dis- 

 covered by Dulong and Petit in 1819, that 

 the specific heat of a metal was inversely 

 proportional to its atomic weight, was more 

 speedily accepted; but even this law did 

 not receive its full application until many 

 years later. To apply either of these laws 

 to chemical theory involved a clearer dis- 

 crimination between atomic weights and 

 equivalents than was possible at the be- 

 ginning. A long period of doubt and con- 

 troversy was to work itself out before the 

 full force of the physical evidence could be 



