IMMUNITY AND SUSCEPTIBILITY. 83 amount of toxin endurance of a resistent animal be repre- sented by x, and any addition to this as y. Then xy would certainly be fatal. If the least quantity of anti- toxin that will protect the animal be expressed by 2, then xy + z is harmless. It is evident, however, that z does not necessarily have any influence upon x^ but only need neutralize y in order to save the animal, and therefore it is obvious that the remaining x in such a mixture could readily destroy another more susceptible animal into which it might be injected. I am of the opinion that the effect of the antitoxin really partakes of the nature of chemic neutralization from the following experiment: let a" represent the least certainly fatal dose of diphtheria toxin for a guinea-pig, and y the least quantity of antitoxin that will protect against it; then x + y is harmless. That 10 x + io.y is also harmless is known to every one accustomed to test antitoxins. I have con- tinued this and have found that 50 •*: + 50 y 100 x + 100 y are also harmless. According to Buchner, the antitoxins differ from the alexins in being new substances in the blood, in being without germicidal or chemical neutralizing power against the toxins, and in being stable compounds which can resist heat to 75° C., can resist a reasonable amount of exposure to light, and which are not altered by decompo- sition of the substances containing them. The antitoxins are specific for one poison only. Ehrlich found that antiricin was powerless against abrin, and vice versd. Diphtheria antitoxin is of no avail against tetanus, and vice versd. The immunity which the antitoxins produce is fuga- cious, varying considerably according to the particular substance employed. As a rule, it is limited to a few months—at least in the case of such antitoxins as we can produce experimentally.