82 REVERSIBLE REACTIONS 



of acid is divided into two parts, of which one combines with 

 the alcohol and the other remains free. As we continue to add 

 successive small amounts, this latter uncombined portion gets 

 greater and greater, so that the addition of successive small 

 volumes use up less and less of the alcohol ; and it is only when 

 there is a great excess of acid that all the alcohol is used up. 

 Theoretically, some always remains. 



The difference may be represented graphically thus : 



a 



d 



b 



FIG. 19. 



The line a b represents the neutralization of a given amount of 

 NaOH by its equivalent of acid. It is a straight line, since each 

 successive addition of equal amounts of acid neutralizes the same 

 amount of alkali. 



The line c d represents the reaction of alcohol and acetic acid, 

 or, to take the example used by Madsen, the neutralization of 

 ammonia by boracic acid. It is a hyperbolic curve. It is almost 

 a straight line to begin with, since each small addition of boracic 

 acid is almost all used up by the ammonia, there being enough 

 NH 4 ions to seize on all the boracic ions which are added. 

 Farther along, however, it changes according to the rules already 

 described, and ultimately approaches infinitely near to the base 

 line, but never reaches it. There are always free boracic acid 

 and free ammonia in the solution, though the amount of the latter 

 is infinitely small when the former is in great excess. 



The first suggestion that the reaction between toxin and anti- 

 toxin might be a reversible one was due to Myers in an investi- 

 gation on the haemolytic action of snake-venom, a substance 



