404 POPULAR SCIENCE MONTHLY. 



himself as an adherent of the new doctrine of types. After his 

 habilitation at Heidelberg, which followed in 1856, came the essay 

 on fulminating mercury, in which the view so important for the 

 future was expressed, that to the three typical combinations of chlor- 

 hydrogen, water, and ammonia, hitherto recognized, might be added 

 a fourth, marsh gas. In the next essay, on binary combinations and 

 the theory of polyatomic radicals, he put forward the conception of 

 mixed types, and first reached the knowledge of various atomicity 

 or valency of the radicals. These researches were continued, and 

 there appeared shortly afterward, in the spring of 1858, the two great 

 treatises which have since exercised so powerful an influence on 

 chemistry — that on the constitution and metamorphoses of chemical 

 combinations, and that on the chemical nature of carbon. In these 

 theses Kekule passed from the valency of the radicals to that of the 

 elements themselves, and showed that the composition of all those 

 compounds that contain one atom of carbon lead to the conclusion 

 that that element is quadrivalent; and that, further, the relations of 

 combination of a complex of carbon atoms are explainable if we 

 suppose that the latter are mutually bound by a certain number of 

 their four unities of attraction. This idea was suggested very care- 

 fully, and the words which the author added at the end of his essay 

 read very curiously to-day: 'Finally, I think I ought still to insist 

 that I attach only little value to speculations of this sort. Since one 

 delving in chemistry must once in a while, in the lack of exact scien- 

 tific principles, content himself with probabilities and temporary hy- 

 potheses, it seems proper to communicate these conceptions, because, 

 as it appears to me, they furnish a simple and fairly general ex- 

 pression for the newest discoveries, and because, therefore, the use 

 of them may assist in the discovery of new facts.' How diffident 

 the words sound, and how far have the expectations been exceeded! 

 "We all know that the theory of valency is to-day the leading guide 

 through all our science; and, although another investigator had a 

 share in its origination, no one disputes that its main foundation and 

 its eminent value in organic chemistry are primarily due to Kekule's 

 idea of the quadrivalency of carbon. 



" After he was called to the University of Ghent, in 1858, Kekule 

 exhibited an indefatigable activity. He began the great series of 

 investigations of the organic acids which, beginning with succinic 

 acid, malic acid, and tartaric acid, and extending afterward to many 

 others, have given complete conclusions as to the nature of these 

 bodies. Contemporaneously, in 1 8 6 0, appeared the first number of the 

 Lehrbuch der organischen Chemie, which was soon followed by other 

 numbers, so that the whole first volume was completed in 1861. All 

 his fellow-chemists who are acquainted with the events of that period 



