366 THE CHEMICAL PROBLEMS OF TO-DAY. 



The researches iu this direction are so uumeroiis aud so toilsome, aud 

 yet the result is so surprisingly simple ! The carbou atom is endowed 

 with four, the oxygen atom with two, the hydrogen atom with one point 

 of attack for the chemical affinity. The cause of the aggregation of the 

 atoms within the molecule lies in the mutual saturation of these units 

 of affinity or valencies. It is the number of valencies which decides 

 the possibility of the existence of a compound. Amongst the legion of 

 imaginable combinations of these three elements only those are capable 

 of existence in which every valency is saturated by that of another 

 atom. Through this knowledge a new method of inquiry was opened, 

 iu particular tor organic chemistry, the immense territory of which for 

 many years seemed totally to absorb the working power of chemists. 

 But then dawned the first signs of a further development. Hardly a 

 decade had elapsed since the general admission of the doctrine of va- 

 lency when a fundamental deepening of the same was announced, which 

 our science owes to two savants, working independently of each other — 

 to Le Bel and van't Hoff". These chemists, cousidi ring those substances 

 which turn the plane of polarization tf light, arrived at views which 

 soon led to a result until then thought to be out of reach, a conception 

 of the aggregation of the atoms within the molecules in space. Thus 

 a field of study was created which van't Hofif called ^'la chimie dans 

 Vespace" and which we now call Stereochemistry. 



It was recognized that the carbon atom stretched out its four valen- 

 cies in definite directions, and this in a symmetrical manner. The 

 combination of a carbon atom with four other atoms, for example, 

 methane, CH4, is representable by the picture of a tetrahedron in the 

 stereometric center of which the carbon atom is situated, while the 

 hydrogen atoms occupy its four corners. 



Numerous cases of isomerism, until then not understood, could be 

 explained in this manner and were regarded as stereo-chemical ones. 

 The cause of oi)tical activity was found to consist in the presence of an 

 a-symmetric carbon atom, that is, one which is combined with four dif- 

 ferent groups. 



Also the stereometric forms of a few simple molecules were consid- 

 ered; it was recognized, e, </., that a compound of three carbon atoms 

 linked together by one bond respectively could not contain those atoms 

 in a straight line, but that they must lie in the angles of a triangle the 

 sides of which form an angle equal to that in which the directions of 

 valency of the carbon atom intersect each other. 



By the applications of these considerations* to more complicated 

 molecules, which contain a chain of atoms closed within itself, Adolph 

 von Baeyer has enlarged our theory in a manner full of consequence. 



Kekule in times past had recognized that carbou shows a particular 

 disposition to form closed chains of six atoms. The discoveries of Bae- 

 yer aud his followers, as well as Fittig's work on lactones, taught that 

 such clostd chains or rings formed of fewer sitoms also exist. But 



