TOXIN AND ANTITOXIN 119 



the formation of ethyl acetate and water; the reaction, however, is 

 incomplete and stops at an equilibrium point at which the reaction 

 mixture contains % gram mol. alcohol, % gram mol. acid, % gram 

 mol. ethyl acetate, and % gram mol. water. If, on the other hand, 

 1 gram mol. of ethyl acetate is mixed with 1 gram mol. of water, a 

 reaction sets in which results in the formation of ethyl alcohol and 

 acetic acid. This change likewise stops in equilibrium at a point at 

 which the composition of the reaction mixture is the same as that 

 already stated." The reaction is thus reversible and may be written: 



C 2 H 6 OH + CHaCOOH ^1 CH 3 COOC 2 H 6 + H 2 



Another example somewhat simpler and more easily brought into 

 analogy with the toxin-antitoxin reaction is that of the dissociation 

 of phosphorus pentachlorid into phosphorus bichlorid and chlorin 

 (see Alexander Smith, "General Chemistry," Century Company, N. 

 Y., 1911, p. 181). 



Here the reaction takes place: 



PC1 5 T-** PCls + C1 2 



Chemical equilibrium is reached when the reaction speed is the same 

 in both directions, and there will be present, at equilibrium, PC1 3 , 

 C1 2 , and PC1 5 . Now the "Law of Mass Action" (Guldberg & 

 Waage) states that reaction goes on at a velocity proportionate to 

 the concentration of the reacting molecules. It is plain, therefore, 

 that at the point at which the reaction takes place with equal veloci- 

 ties in both directions, that is, at the equilibrium point, a very defi- 

 nite relation of molecular concentrations must obtain, and this rela- 

 tion can be expressed as a formula. For the example given above 

 this may be written as follows : 



Cone. PC1 3 X Cone. Cl ^ , ,. 



7^ s = K (constant) 



Cone. PCI 5 



This formula is expressed in words by Alexander Smith as follows: 

 "If we change the amount of the pentachlorid placed in the vessel, 

 or if we use amounts of chlorin and trichlorid which are not equiv- 

 alent, the numerical value at equilibrium of each concentration will, 

 of course, be different, but the product of the concentrations of tri- 

 chlorid and chlorin, divided by the concentration of the pentachlorid, 

 will always give the same numerical value for (K), the constant, at 

 the same temperature." 



Now to return to the application of these facts to the neutraliza- 

 tion of toxin by antitoxin, if the reaction is one analogous to that of 

 a strong acid and alkali, as cited above in the case of H 2 SO 4 and 



