JACOBUS HENRICUS VAN'T HOFF. v 



rical form in three dimensions in space in which a central object is 

 surrounded symmetrically by four things of the same kind. Thus 

 arose the conception of the " tetrahedral carbon atom." Pasteur had 

 been studying the property possessed by certain liquids of rotat'ng the 

 plane of polarization of a beam of polarized light passed through 

 them. He had reached the conclusion that in order that a liquid 

 should have this property — be " optically active "■ — its molecules must 

 possess some kind of asymmetry. Further than this Pasteur could 

 not go. The solution of the problem of optical activity remained 

 for Van't HoiT. 



He examined the constitution of all of the optically active com- 

 pounds of carbon then known, and found that they all contain at 

 least one carbon atom in combination with four different atoms or 

 groups : and this applies to every optically active compound of carbon 

 known today ; and these number more than a thousand. Van't Hoff 

 simply extended his theory of the " tetrahedral carbon atom " to 

 that of the "asymmetric tetrahedral carbon atom" and the problem 

 of optical activity was solved. 



This was the beginning of the stereochemistry of carbon, from 

 which the stereochemistry of several other elements is the outgrowth ; 

 and it is not an exaggeration to say that the tetrahedral carbon atom 

 has been the guiding thought in organic chemistry for the past thirty 

 years. 



Shortly after the appearance of the little paper in Dutch Van't 

 Hoff published his book on organic chemistry "Ansichten iiber die 

 organische Chemie." In this work he attempted to systematize or- 

 ganic chemistry, and especially to place it upon a quantitative basis. 

 He was impressed with the purely qualitative nature of organic 

 chemistry as exemplified by the Kekule school. Certain substances 

 were brought together under certain conditions and certain " yields " 

 were obtained. Very little had been done up to that time towards 

 measuring the velocity of reactions, or the conditions under which 

 chemical equilibrium was reached. These were the problems to 

 which Van't Hoff next turned, and the results of his studies in this 

 field constitute his second great contribution to chemical science. 



It had long been known that mass or relative quantity of the 



