LECTURE VI.] HISTORY OF CHEMISTRY. 103 



initiation of the numbers told in its favour. In particular, the 

 atomic weights of the best known elements, such as, oxygen, 

 hydrogen, nitrogen, carbon, (chlorine?), bromine, iodine, etc., 

 appeared to be in harmony with it. Stas has proved, however, 

 by means of experiments in which the highest degree of accu- 

 racy and completeness was attained, 47 that even in the case of 

 those elements which appeared in accord with it, the hypothesis 

 does not in any instance hold rigidly, and can only be looked 

 upon as an approximation. 



The highly important theory more recently established by 

 Newlands, Lothar Meyer, and, especially, Mendelejeff, dealing 

 with the periodic relation of the properties of the elements to 

 the magnitudes of their atomic weights, can only be entered 

 upon in a subsequent lecture. 



In France, the law of volumes in its most extended sense 

 became the basis of the atomic considerations. It was Dumas, 

 especially, who took up a very decided position in this connec- 

 tion. . He shows that the conception of the equivalent cannot 

 be employed as the basis of a system, because it loses its signifi- 

 cance when it is extended further than to acids, to bases, and to 

 other substances which closely resemble each other (oxides and 

 sulphides) ; and, especially, that it becomes quite vague when 

 the attempt is made to identify the equivalent with the com- 

 bining weight, 48 since very many substances can combine in 

 several proportions. Thus, for example, 8 parts of copper are 

 combined with i part of oxygen in cuprous oxide, while for 

 8 parts of copper, 2 parts of oxygen are contained in cupric 

 oxide. Calculated from these numbers, the equivalent (com- 

 bining weight) of copper, referred to that of oxygen as unity, is 

 8 or 4. 



Dumas believes that, by adopting Avogadro's hypothesis as 

 a basis, he has obtained a sure guide in considerations regard- 

 ing atomic weights. He assumes that in equal volumes of all 



47 Recherches sur les lois des proportions chimiquesetc., Bruxelles 1865, 

 and Recherches sur les rapports reciproques des poids atomiques, 1860. 

 48 Dumas, Traite de Chimie appliquee aux arts. Paris (1828-46). 



