THE ATOMIC THEOEY. 249 



stiinces, or to rational luultiplcs thereof. In 1811 Avogadro general- 

 ized the new evidence and broug'ht forward the great law which is 

 now known by his name. Equal volumes of gases, under like condi- 

 tions of temperature 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 mole- 

 cules, 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 usualh^ 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 1)}^ them, but its decomposition into smaller and 

 simpler particles of oxygen and hydrogen, the chemical atoms, was far 

 beyond the range of their knowledge. That the distinction is not 

 alwavs borne in mind by physicists is illustrated I)}- the fact that in 

 Clerk Maxwell's article "Atom," in the Encyclopa3dia Britannica, 

 Dalton is not even mentioned, and the phenomena there selected for 

 discussion are molecular only. 



Maxwell was surely not ignorant of the dili'erence between atoms 

 and molecules, but his knowledge had not reached the point of com- 

 plete realization. His thought was of molecules, and so Maxwell 

 unconsciously neglected the real subject of his chapter — the atom. Of 

 late 3"ears man}^ essays upon the atomic theory have ])een written 

 from the physical side, and few of them have been free from this par- 

 ticular ambiguity. At first a similar error was committed by chem- 

 ists, who paid small attention to Avogadro's law, and so the latter 

 failed to exert unich influence upon chemical thought until more than 

 forty years after its promulgation. The relation discovered by Dulong 

 and Petit in 1819, that the specific heat of a metal was inversel}^ pro- 

 portional to its atomic weight, was more speedily accepted; but even 

 this law did not receive its full application until many j^ears later. 

 To ai)plv either of these laws to chemical theory involved a clearer 

 discrimination between atomic weights and equivalents than was pos- 

 sibk> at the beginning. A long period of doubt and controversy was 

 to work itself out before the full force of the physical evidence could 

 be appreciated. Mltscherlich's researches upon isomorphism were 

 more fortunate, and gave imniediate help in the determination of 

 atomic weights and the settlement of formula?. For the moment we 

 need only note tiiat the chemical atom was the underlying conception 

 by means of which all these lines of testimony were to be unified. 



From Dalton and (lay-Lussac to Frankland and Cannizzaro was a 

 time of fermentation, discussion, and discovery. In chemistr}-, con- 

 trarj' to the sajangof the preacher, ther(> were many new things under 

 the sun, and some of the discoveries were most suggestive. First, it 



