Il8 HISTORY OF CHEMISTRY. [LECTURE VII. 



gas, when he discovered a hydrocarbon that behaved very like 

 ethylene; but it yielded no chloride of carbon when mixed 

 with chlorine and exposed to sunlight, and it possessed, more- 

 over, a density double that of ethylene. 24 Further, an investi- 

 gation of phosphoric acid was made at this period by Clark, 

 who, through neglecting the fact that the salts contained water, 

 was led to the opinion that there were two phosphoric acids, 

 with different properties but with the same composition. 25 

 Berzelius had previously observed the same thing with respect 

 to stannic acid. 26 He also showed in 1830 that the acid pro- 

 duced along with tartaric acid during the manufacture of the 

 latter, had the same composition as tartaric acid. He calls 

 the new substance racemicacid (Drufsyra. Traubensiiure.}, and 

 introduces the word isomer to designate substances of this 

 kind. According to him, this word is only to be applied to 

 compounds possessing the same composition and the same 

 atomic weight, but with dissimilar properties. 27 A year later, 

 Berzelius designates as polymerism the phenomena observed 

 by Faraday respecting the hydrocarbons. This name embraces 

 those cases where the same composition is accompanied by 

 dissimilar properties and different atomic weights. 28 Metameric 

 substances, on the other hand, are those which possess the 

 same composition, the same atomic weight, and dissimilar 

 properties, when the difference can be explained by a different 

 arrangement of the atoms i.e. by a different constitution. 29 

 As an example, Berzelius very appropriately chooses stannous 

 sulphate and stannic sulphite, which he writes : SnO + SO 3 and 

 SnO 2 + SO Z . 



At that time, the different modifications of an element were 

 also regarded as cases of isomerism ; and it was only in 1841 



24 I mention here, in passing, that Faraday on the occasion of this 

 investigation also discovered benzene. 25 Edinburgh Journal of Science. 7, 

 298; Schweigger's Journal. 57, 421. 26 Ibid. 6, 284. 27 Pogg. Ann. 19,' 

 305. 2S It appears from this that Berzelius at that time regarded the 

 vapour densities of compounds also as a guide to their atomic weights. 

 29 Berzelius, Jahresbericht, 1833, 6 3- 



