CHEMISTRY. 77 



by Reich and Richter in 1863 ; and gallium, by Lecoq de Boisbaudran 

 in 1875. 



As regards theoretical views, the atomic theory, the foundation of 

 scienti6c chemistry, had been propounded by Dalton (1804-1808). 

 The three laws which have been chiefly instrumental in establishing 

 the true atomic weights of the elements the law of Avogadro (1811), 

 that equal volumes of gases under the same conditions of temperature 

 and pressure contain equal numbers of molecules ; the law of Dulong 

 and Petit (1819), that the capacities for heat of the atoms of the 

 various elements are equal ; and Mitscherlich's law of isomorphism 

 (1819), according to which equal numbers of atoms of elements be- 

 longing to the same class may replace each other in a compound with- 

 out altering the crystalline form of the latter, had been enunciated 

 in quick succession; but the true application of these three laws, 

 though in every case distinctly stated by the discoverers, failed to be 

 generally made, and it was not till the rectification of the atomic 

 weights by Cannizzaro, in 1858, that these important disco veiies bore 

 fruit. 



In organic chemistry the views most generally held about the 

 year 1830 were expressed in the radical theory of Berzelius. This 

 theory, which was first stated in its electro-chemical and dualistic 

 form by its author in 1817, received a further development at his 

 hands in 1834 after the discovery of the benzoyl-radical by Liebigand 

 "Wohler. In the same year (1834), however, a discovery was made by 

 Dumas, which was destined profoundly to modify the electro-chemical 

 portion of the theory, and even to overthrow the form of it put forth 

 by Berzelius. Dumas showed that an electro-negative element, such 

 as chlorine, might replace, atom for atom, an electro-positive element 

 like hydrogen, in some cases without much alteration in the character 

 of the compound. This law of substitution has formed a necessary 

 portion of every chemical theory which has been proposed since its 

 discovery, and its importance has increased with the progress of the 

 science. It would take too long to enumerate all the theoretical views 

 which have prevailed at various times during the past fifty years ; 

 but the theory which along with the radical theory has exercised most 

 influence on the development of the views now held, is the theory of 

 types, first stated by Dumas (1839) and developed in a different form 

 and amalgamated with the radical theory by Gerhardt and Williamson 

 (1848-1852). It is, however, the less necessary to refer in detail to 

 these views, seeing that in the now prevailing theory of atomicity we 

 possess a generalisation which, while greatly extending the scope of 



