476 SCIENCE PROGRESS 



words, we are exchanging a small light group for a large 

 heavy one. 



Let us next consider what takes place when an acid reacts 

 with an alcohol. Obviously the hydroxyl group of the alcohol 

 must come within range of the carboxyl group of the acid before 

 any interaction can take place, since the ester is formed by the 

 elimination of water between these two groups. Now, if we 

 could interpose some barrier between these two radicles, we 

 should hinder the esterification ; and the more effective the 

 barrier was, the less ester would be formed. Such a barrier 

 is found in the methyl groups of the substituted acetic acids. 

 According to the current theories of chemical action, a certain 

 amount of free space must exist around the hydroxyl group 

 before the carboxyl radicle can come within reaction range. 

 The light hydrogen atoms are easily thrust aside by the heavy 

 carboxyl group, and therefore the esterification of acetic acid is 

 easy. When we replace the light hydrogen atoms by the 

 heavier methyl radicles, however, the resistance to the entrance 

 of the carboxyl radicle becomes serious, and consequently much 

 less ester results. The following analogy may help to make the 

 matter clearer : Suppose a man to be desirous of passing 

 through a certain door, guarded by children. He would be 

 able to pass because his strength would be greater than that 

 of the guards ; but if the children became men, his chances of 

 passing the door would be very considerably reduced. 



This steric hindrance, as it is called, has been observed in 

 nearly every class of reaction ; and there seems to be no doubt 

 that the substitution of one group for another may influence 

 the course of a reaction to an extent which seems quite out of 

 comparison with the slight change in the chemical character 

 of the substance which the substitution produces. We must 

 now consider an application of this view to the question of 

 vital action. 



On Ehrlich's theory of antitoxins there are two sets of 

 molecules in living tissue : those on the outer surface of the 

 cells being concerned with the nutrition process, while those 

 within are employed in cell metabolism. If the outer, or 

 peripheral, molecules can be put out of action, the cell lacks 

 nutrition and eventually dies. Ehrlich supposes that the 

 peripheral molecules have active side-chains serving to anchor 

 suitable molecules which may be carried past them ; and in 



