ANALOGY BETWEEN REACTIONS 517 



The difference between the L and L+ dose of a toxin also has an an- 

 alogy in the reaction of simple colloidal substances. Thus, Bilty has used 

 ferric hydroxid, which neutralizes arsenic trioxid (the antidote for acute 

 arsenical poisoning), and found that the addition of one lethal dose of 

 arsenic to a neutral mixture of the two did not render the mixture toxic, 

 but that several lethal doses were required, just as it is necessary to add 

 several instead of one lethal dose of diphtheria toxin to the L dose. 



Thus it would appear that the neutralization of a toxin by an anto- 

 toxin has analogies among the known and simple colloidal reactions. 

 One objection to placing the toxin-antitoxin reaction upon a colloidal 

 basis is that both have the same electric charge, i. e., both move toward 

 the cathode, and, as we have seen, for the neutralization and precipita- 

 tion of colloids the solution of colloids should be of opposite sign. It 

 must be remembered, however, that toxins and antitoxins react in very 

 complex fluids containing other substances consisting of both colloids 

 and electrolytes, and until the electric charge of these in pure form is 

 determined, the apparent similar electric charge of toxin and antitoxin 

 can hardly outweigh the otherwise remarkable analogy it bears to col- 

 loidal reactions. 



2. Agglutinins and Precipitins. Various theories in explanation of 

 the phenomenon of agglutination have been described in a previous 

 chapter. The theory of Bordet appears to be best, and is based upon 

 certain principles of colloidal chemistry. When bacteria are suspended 

 in a fluid free from salt, agglutination does not take place because the 

 bacteria carry a similar negative charge of electricity. When, however, 

 ions of positive charge are added, as, e. g., sodium chlorid, the bacteria or 

 other cells are repelled and coalesce to form masses, according to the 

 laws of surface tension, in an effort to protect themselves. Larger 

 masses may be formed that finally come within the influence of gravity 

 and are deposited at the bottom of the test-tube. According to the 

 same laws, the addition of agglutinin removes the negative charge of 

 bacteria or other cells, with the consequent formation of clumps and 

 masses. Similar phenomena may be observed in the precipitation of 

 colloidal suspensions of clay in- distilled water by the addition of a salt. 



Solutions of inorganic colloids, as, for example, that of silicic acid, 

 may agglutinate red corpuscles; bacteria, such as suspensions of typhoid 

 and colon bacilli, may be agglutinated by solutions of the ferric salts. 



Just as an excess of one colloid solution will charge masses of the 

 other, resulting in a repelling action and breaking up of th.e agglutinated 

 clumps, so the addition of an excess of agglutinin is found to prevent 



