212 INFECTION AND IMMUNITY 



a bacteriologist who was once a pupil of Ehrlich, have recently opposed 

 Ehrlich's theory on grounds of physical chemistry. 



Modern theories of solution maintain that substances in solution 

 are broken up into their atoms or atom-groups, known as ions. Thus, 

 NaCl in solution would be " dissociated " into its Na ion and its Cl ion, 

 the completeness of the dissociation depending upon the concentra- 

 tion of the solution. A solution of NaCl, therefore, contains, according 

 to this view, three substances, NaCl undissociated and free ions of Na 

 and Cl, the relative quantities of the three present in any given solution 

 being calculable, and depend upon a law known as the law of mass- 

 action of Guldberg and Waage. These free ions are the elements, there- 

 fore, which are active in the formation of further chemical combination. 

 When a strong acid, in solution, acts upon a base, say HC1 upon am- 

 monia (NH 3 ), strong acid having the property of quite complete dis- 

 sociation in relatively concentrated solutions, little or no ammonia 

 would remain unbound. A weak acid, like boric acid, however, not being 

 as completely dissociated, would leave some ammonia uncombined even 

 after more than quantitatively sufficient boric acid had been added. 

 Arrhenius and Madsen, on the basis of careful researches into the re- 

 action between tetanolysin and its antibody, believe that toxin and anti- 

 toxin possess weak chemical avidity for each other, their interaction 

 being comparable to that taking place between a weak acid and a base. 

 Toxin-antitoxin solutions, therefore, would contain the neutral com- 

 pound, but at the same time uncombined toxin and antitoxin. The 

 qualities which Ehrlich ascribes to toxon, they believe, are due to 

 the unbound toxin present in such mixtures. In careful studies 

 in which they inhibited the hemolytic action of ammonia by gradual 

 addition of boric acid, they were able to show complete parallelism 

 between the conditions governing this neutralization and those con- 

 cerned in their tetanus experiments. Their explanation has the 

 advantage of extreme simplicity over that of Ehrlich, but since the 

 differences of opinion are now the subject of active experimental 

 controversy, a critical discussion must rest until further facts are 

 revealed. 



The Side-Chain Theory. We have seen that the extensive researches 

 of Ehrlich into the nature of the toxin-antitoxin reaction led him to 

 believe that the two bodies underwent chemical union, forming a neu- 

 tral compound. The strictly specific character of such reactions, further- 

 more, diphtheria antitoxin binding only diphtheria toxin, tetanus 

 antitoxin onlv tetanus toxin, etc., led him to assume that the chemical 



