LECTURE xiii.] HISTORY OF CHEMISTRY. 251 



carbon ; but this is only referred to incidentally, and does not 

 lead him to any further conclusions. 



At length, in the spring of 1858, the paper appears which 

 has become of such fundamental importance in chemistry. 7 

 In this paper Kekule begins to direct attention to the necessity 

 of studying the nature of the elements. This alone, in his 

 opinion, can lead to an explanation of the valency of the 

 radicals. As regards organic chemistry, the chief part in such 

 considerations is played by carbon ; and, consequently, the 

 properties of this element are subjected by Kekule to a very 

 minute examination. " When the simplest compounds of this 

 element are considered (marsh gas, methyl chloride, chloride 

 of carbon, chloroform, carbonic acid, phosgene gas, sulphide of 

 carbon, hydrocyanic acid, etc.) it is perceived that that quantity 

 of carbon which chemists have recognised as the smallest pos- 

 sible, that is, as an atom, always unites with four atoms of a 

 monatomic or with two atoms of a diatomic element ; that, in 

 general, the sum of the chemical units of the elements united 

 with one atom of carbon is four. This leads us to the opinion 

 that carbon is tetratomic (or tetrabasic)." The hypothesis of 

 the linking of carbon atoms also appears now, and is dealt with 

 in a very detailed manner. " In the cases of substances which 

 contain several atoms of carbon, it must be assumed that at 

 least some of the .atoms are in the same way held in the com- 

 pound by the affinity of carbon, and that the carbon atoms 

 attach themselves to one another, whereby a part of the affinity 

 of the one is naturally engaged with an equal part of the affinity 

 of the other. 



"The simplest, and, consequently, the most probable case 

 of such an attachment of two carbon atoms is that in which 

 one unit of affinity of the one atom is united with one of the 

 other. Of the 2x4 affinities of the two carbon atoms, two are 

 therefore employed in holding the two atoms themselves to- 

 gether ; six thus remain over which can be held by atoms of 

 other elements." 



7 Annalen. 106, 129. 



