242 



SCIENCE. 



[N. S. Vol. XI. No. 288. 



he left Amsterdam to accept a chair of 

 Physical Chemistry at the University of 

 Berlin, which had been founded especially 

 for him. 



Van't Hoff's contributions to science fall 

 into three classes ; or we may saj', in brief, 

 he has done three things. His earliest work 

 of importance had to do with the spatial 

 arrangement of the atoms in the molecule. 

 In the same year in which he obtained the 

 degree of Doctor of Philosophy, he pub- 

 lished a small pamphlet of eleven pages, 

 which is the beginning of all stereochemis- 

 try, la the following year it was enlarged 

 to forty- four pages, and published in French 

 under the title, " La chimie dans I'espace." 

 It was translated into German two years 

 later, with an enthusiastic preface by 

 Wislicenus. 



The attitude of Kolbe, who utilized his 

 position to belittle not only the work but 

 also its author, is familiar to every one. 

 Indeed, so familiar, that Kolbe's reputation 

 for liberality is not materially increased by 

 the criticism which he offered of this work. 



Van't Hoif attempted, in this work, to 

 formulate a rational theory of the arrange- 

 ment of tbe atoms in the molecules of cer- 

 tain substances. Let us consider the very 

 simple compound of carbon and hydrogen, 

 CHj. All the properties of this substance 

 show that it is a symmetrical compound, 

 every hydrogen atom bearing exactly the 

 same relation to the molecule. By what 

 possible geometrical configuration in three 

 dimensions can this be expressed? Evi- 

 dently by the regular tetrahedron, and by 

 this alone. If we represent the carbon 

 atom as being placed at the center of a 

 regular tetrahedron, and the four hydrogen 

 atoms at the four corners, or in the four 

 solid angles of the tetrahedron, we have a 

 perfectly symmetrical configuration. This 

 was pointed out by Van't Hoff, and has 

 come to be known as the ' theory of the 

 tetrahedral carbon atom.' 



One application of this theory will serve 

 to show how it has been of service in ad- 

 vancing our knowledge of organic substance. 

 A comparatively large number of com- 

 pounds were known, which, in the liquid 

 state or in solution, would rotate the plane 

 of polarization when a beam of polarized 

 light was passed through them. In some 

 cases the rotation was to the right, in other 

 cases to the left. Further, it was very 

 probable that for every substance which 

 rotates the plane of polarization in one 

 direction, there is a substance of the same 

 composition rotating the plane in the oppo- 

 site direction. How could these facts be 

 interpreted in terms of the theory of the 

 tetrahedral carbon atom ? 



Van't Hoff pointed out that every one of 

 these so-called optically active substances 

 contains a carbon atom in combination with 

 four different atoms or groups. It is only 

 when the four atoms or groups at the solid 

 angles of the tetrahedron are all different, 

 that two configurations are possible. If 

 any two of the atoms or groups are the same, 

 it is impossible to construct two tetrahedra 

 which shall differ from one another. But 

 if the four atoms or groups are all different, 

 two tetrahedra can be constructed which 

 bear the relation to each other of an object 

 and its image in a mirror. These two con- 

 figurations represent the two substances 

 which have the same composition, but are 

 optically active in opposite senses. When 

 this theory of ' the asj'mmetric carbon 

 atom ' is applied to all of the facts known 

 concerning optically active substances, we 

 find that out of the seven hundred optically 

 active compounds, there is only one which 

 may be an exception to it. But this sub- 

 stance is so complex that its constitution is 

 far from settled. 



This is but one application of the theory 

 of the tetrahedral carbon atom. This theory 

 has thrown entirely new light on the mean- 

 ing of isomerism in organic chemistry. By 



