92 PROGRESS OF SCIENCE IN THE CENTURY. 



By improved methods of determining the specific 

 gravity of gases and vapours, " the all-important 

 knowledge of the relative weights of the atoms and 

 molecules of elements and compounds has been im- 

 mensely advanced" (E. von Meyer, p. 441). From 

 the study of anomalous vapour-densities, H. de St. 

 Claire Deville discovered in 1857 the fact of " dis- 

 association " or the gradual decomposition of a com- 

 pound with rise of temperature, the starting-point 

 for another series of important investigations. 



Though confirmed by similar conclusions (Davy, 

 1812, Ampere, 1814), Avogadro's hypothesis: 

 " Equal volumes, equal number of particles " was 

 not appreciated until the establishment of the kinetic 

 theory of gases (q.v.), and " no substantial chemical 

 reasons for its adoption were adduced until the year 

 1846, when Laurent published his work on the law 

 of even numbers of atoms and the nature of the ele- 

 ments in the free state." * 



Further Influence of Physical Researches. 

 When the century was about half over, the doctrine 

 of fixed and multiple proportions was generally ac- 

 cepted (with some saving clauses for not-solid com- 

 pounds), but the conception of atoms which lay be- 

 hind this doctrine was looked at more cautiously. 

 The careless may have believed in the physical exist- 

 ence of these smallest indivisible particles, but this 

 was certainly not the general belief. And even as a 

 symbolism, as an alphabet, as a means of notation, 

 there were many chemists who doubted if the atom- 

 concept was indispensable or even legitimate. Cor- 

 roboration had to come from an independent source, 

 and it came from the physicists, more especially 



* Prof.. R. Meldola, Address, Section B, Rep. Brit. Ass. for 

 1895, p. 639. 



