LECTURE VII.] HISTORY OF CHEMISTRY. 119 



that Berzelius introduced the word allotropy to designate such 

 cases. 30 A number of examples belonging to this class were 

 already known ; one of the most interesting being carbon, in 

 the forms of diamond, graphite, and soot. 



It will be understood that the idea of metamerism could 

 only be introduced after it had become possible to entertain 

 any conception of the constitution of a substance ; while, on 

 the other hand, the phenomena of isomerism necessarily led 

 chemists to hypotheses respecting the mode of arrangement of 

 the atoms. It is well known that there was in existence at that 

 time a mode of regarding the facts which Berzelius endeavoured 

 to extend more and more. I refer to dualism, which I have 

 already had occasion to mention several times, and the conse- 

 quences of which I shall now state more definitely. 



The phenomena of combustion had led Lavoisier to assume, 

 in so far as it was possible to do so, that substances consisted 

 of two parts. This way of regarding them was very advan- 

 tageous and clear in the case of salts, which were looked upon 

 as composed of base and acid. This view was in agreement 

 with their whole behaviour, and rendered it possible to con- 

 sider them all from one common standpoint. The arguments 

 of Gay-Lussac and Thenard against the elementary nature of 

 chlorine, which have been stated in a previous lecture, 31 prove 

 how deeply these ideas had become rooted, and how firmly it 

 was the custom to base conclusions upon them. 



After the existence of the so-called hydrogen acids (i.e. 

 of acids which do not contain oxygen) had been generally 

 admitted, various opinions arose respecting the nature of their 

 salts. A few investigators (Davy and Dulong, for example) 

 regarded them as compounds of metals, just as they regarded 

 other salts; 32 but this view met with little approval at that 

 time. Others remained true to the earlier conception, and 

 with them common salt was still muriate of soda, which had 

 the peculiarity, however, that it gave off its " water." Others, 



30 Berzelius, Jahresbericht, 1841, Part II. 13. 3l Seep. 81. 32 See 

 p. 83. 



