254 HISTORY OF CHEMISTRY. [LECTURE XIII. 



only correct one. Couper considers it necessary to study first 

 the properties of the elements ; and these he regards as : 



1. Elective affinity, 



2. Degree affinity. 



The latter governs the limits of combining capacity, and 

 coincides approximately with what we now call valency or 

 atomicity. In his further consideration of the subject Couper 

 confines himself to the determination of the degree affinity 

 of carbon, and believes that, by means of it, he can elucidate 

 organic compounds. There are two essential properties of 

 this element which serve to characterise it : *i, It combines 

 only with an even number of hydrogen atoms ; and 2, It 

 combines with itself. The latter assertion is justified by 

 reference to compounds containing carbon. Hydrogen, 

 oxygen, etc., can be removed from these compounds and their 

 places can be taken by chlorine without interfering with the 

 linking, consequently the cause of this linking cannot be 

 sought for in the atoms capable of substitution. The maximum 

 number of atoms united to one carbon atom is four, and from 

 this Couper obtains for what we should now call saturated 

 organic compounds the expression : 



These observations are sufficient to enable us to understand 

 Couper's formulae, of which I shall quote a few examples [C = 1 2, 

 = 8]:-" 



. a | 3 3 | 



TT /"V TT TT 



C O OH C O OH C O O" C 



Alcohol. Acetic acid. Ether. 



11 Couper makes curious hypotheses with respect to the properties of the 

 oxygen atom, obviously in order that he may not be obliged to assume in 

 the formation of salts the replacement of the hydrogen by metal (and therefore 

 a reduction of the oxide). According to him O = 8 is bivalent, but one 

 valence must always be satisfied by oxygen. The limit of the valency of 

 nitrogen is adopted as 5. (In the paper in Ann. Chim., the formulae are 

 written on the basis of C 6.) 



