430 



SCIENTIFIC THOUGHT. 



mation of the molecule of hydrochloric acid depended upon 

 an exchange of places of the atomic constituents in the 

 molecules of the elementary substances, an atom of chlor- 

 ine being substituted for an atom of hydrogen in the hydro- 

 gen molecule, and vice versd in the chlorine molecule. 



About the middle of this century the conviction was thus 

 firmly established in the minds of chemical philosophers 

 that the simple symbolism by which Dalton and Berzelius 

 expressed chemical combinations and processes was in- 

 sufficient for the purpose of systematically arranging the 



nition of the difference between 

 atom and molecule belongs also 

 to Gerhardt, who emphasised a fact 

 known already to Berzelius viz., 

 that hydrogen according to his 

 notation appeared to combine with 

 other bodies always in paired atoms. 

 This fact remained unnoticed if the 

 atomic number of hydrogen was 

 put at 1, oxygen at 8, as was done by 

 English chemists and reintroduced 

 by Gmelin. Berzelius did not attach 

 a fundamental importance to this 

 fact. Blomstrand (' Die Chemie der 

 Jetztzeit,' 1869, p. 30) has shown 

 that this originated in his clinging 

 to Lavoisier's oxygen theory. Oxy- 

 gen was made the centre and meas- 

 ure of everything in chemistry, also 

 of the equivalence of substances : 

 Berzelius thus started from a 

 unit which was too large, and with 

 which the smaller value of hydrogen 

 could not be measured. Gerhardt 

 fully recognised the importance 

 of this fact ; showed in many 

 examples that the. combining or 

 atomic weight of hydrogen had 

 been fixed too high ; and proposed 

 to halve most of the organic for- 

 mulae. In this way he proposed 

 to bring harmony into the theory 

 of combining volumes and the 

 atomic theory. He partially suc- 

 ceeded in doing so, although in the 



case of inorganic elements he went 

 too far. This important step, 

 which has been extolled by some, 

 and depreciated by other historians 

 of chemistry, is lucidly expounded 

 by Rau in his 'Theorien der mo- 

 dernen Chemie' (vol. ii. p. 107, &c.) 

 Wurtz ('The"orie atomique,' p. 64) 

 considers Gerhardt's influence as a 

 reform, and alludes to it as bringing 

 again into view the hypothesis of 

 Avogadro : " Voila le theme d'Avo- 

 gadro et d'Ampere, qui revient a 

 1'horizon, comme une e"toile diri- 

 geante, apres une longue e"clipse. 

 Et pourtant on ne peut pas dire 

 qu'elle ait ete" pour Gerhardt, a 

 cette e"poque du moins, un guide 

 exclusif. Les considerations mai- 

 tresses qu'il a invoque"es sont plutot 

 d'ordre purement chimique. Elles 

 e"taient justes, et il s'est trouve" 

 qu'elles concordaient avec une ide"e 

 egalement juste et qui etait tomb^e 

 dans 1'oubli. La distinction entre 

 deux especes de petites particules, 

 molecules et atomes, qu'Avogadro 

 et Ampere avaient introduite in- 

 utilement dans la science, que M. 

 Dumas avait essaye de faire revivre 

 dans sa Philosophic chimique, cette 

 distinction e"tait peut - etre faite 

 dans 1'esprit de Gerhardt, mais elle 

 n'apparaissait pas encore dans son 



