428 



SCIENTIFIC THOUGHT. 



volume as did each of the equivalent quantities, hydrogen 

 and chlorine, out of which it was compounded, and it 

 appeared that accordingly double the number of atoms 

 were condensed into the same volume. To explain this, 

 and yet maintain his original hypothesis, Avogadro was 

 forced into the conception of compound atoms or particles 

 i.e., into the assumption that the smallest independent 

 particles need not be the elementary atoms of hydrogen 

 and chlorine themselves, but might be made up of two 

 or more of such atoms, chemically connected in such 

 a way that the expansion of the gas under increasing 

 temperature or decreasing pressure did not affect this 

 complex of elementary particles. 1 Such a compound 



1 Avogadro published his memoir 

 in the ' Journal de Physique ' in 

 1811, and Ampere expounded simi- 

 lar views three years later in the 

 form of a letter to Berthollet in the 

 ' Annales de Chimie. ' Neither the 

 celebrity of Ampere nor the ex- 

 haustive explanations of Avogadro, 

 who was then an unknown author, 

 prevented this hypothesis, which 

 is now looked upon as a corner- 

 stone of the atomic view, from 

 falling into oblivion. Whewell 

 does not mention it. Even Kopp, 

 whose labours for many years 

 covered a field little cultivated by 

 most other chemists, that of physi- 

 cal chemistry, makes no mention 

 of Avogadro's and Ampere's hypo- 

 thesis in his great work on the 

 History of Chemistry, published 

 between the years 1843 and 1847. 

 In his later work ('Die Entwickelung 

 der Chemie,' 1873) he enters ela- 

 borately into the causes which made 

 chemical philosophers overlook so 

 valuable a suggestion (p. 353, &c.) 

 Like Whewell's History, Poggen- 

 dorf's Dictionary (1863) was sil- 

 ent about Avogadro. The distinc- 



tion between molecules and atoms 

 seemed to complicate matters ; be- 

 sides, the new hypothesis was not 

 launched in conjunction with any 

 new experimental discoveries, as 

 had been the case with Dalton's, 

 Davy's, and Gay Lussac's theories. 

 The first who again drew attention 

 to the subject was Dumas, who in 

 1826 began his investigations re- 

 garding the specific weight of 

 vapours i.e., of bodies in a gaseous 

 state. He there drew attention to 

 the necessity of distinguishing be- 

 tween chemical and physical par- 

 ticles, but he does not yet con- 

 sistently use the terms atom and 

 molecule to denote the former and 

 the latter. In the meantime, how- 

 ever, a very important step had 

 been taken in the development of 

 the atomic view. In 1 819 Dulong and 

 Petit published their experimental 

 researches concerning the specific 

 heat of a large number of element- 

 ary bodies i.e., the measured quan- 

 tities of heat (compared with a 

 standard substance) which were 

 required to raise a number of 

 metals by one degree in tempera- 



