266 INFECTION AND RESISTANCE 



show that the relative quantities of the two reacting bodies are quite 

 as significant as their chemical or physical constitution in determin- 

 ing the occurrence of flocculation. This, according to Bechold, Bil- 

 litzer, 46 and others depends upon the fact that the reason for floccu- 

 lation is one of electrical charge. One hydrosol say arsenic tri- / 

 sulphid can be flocculated by the oppositely charged colloidal ahu / 

 minium hydroxid, but this will occur only when the quantitative^ 

 relations are properly adjusted. If one or the other is in excess, no 

 flocculation may occur, and, if subjected to a direct current, both 

 colloids, though ordinarily wandering in opposite directions, will 

 now wander in that of the one which is now present in the largest 

 amount. We will not elaborate here upon the causes for this, since 

 they have been indicated in the section on Agglutinins, and are set 

 forth more accurately by Prof. Young in the special chapter on Col- 

 loids. 



This effect of quantitative proportions would explain not only 

 the absence of precipitation in the presence of too much antigen, but 

 also the converse phenomenon, already mentioned, that precipitation 

 may be inhibited when the precipitin is in excess. 



The fact that heated precipitating serum when added to its an- 

 tigen not only does not cause flocculation, but may even prevent sub- 

 sequent precipitation by active precipitin, also finds its analogy in 

 colloidal reactions in the so-called protective colloids. Thus arsenic 

 trisulphid may be protected from precipitation by gelatin, if a small 

 amount of gum arabic is added, and the analogy has been brought 

 even closer by Forges, 47 who showed that heated serum will protect 

 mastic suspension from precipitation by normal serum. This obser- 

 vation of Forges is so closely similar to the results obtained by Kraus 

 and v. Pirquet and others on the inhibition of precipitation by heated 

 precipitating serum that it would seem, on first consideration, effec- 

 tually to refute the conception of "precipitoids." 



However, it does not explain the specificity of such inhibition on 

 the part of heated precipitating serum, as reported by Kraus and v. 

 Pirquet, an observation which is one of the strongest arguments in 

 favor of the derivation of the inhibiting factor from the specific 

 precipitin (a precipitoid) , 48 



In spite of the strong evidence in favor of the colloidal inter- 

 pretations, such contrary evidence, brought forward by careful and 



46 Billitzer. Cited from Bechold, "Die Kolloide, etc.," p. 79. 



47 Forges. Chapter on "Colloids and Lipoids" in "Kraus u. Levaditi 

 Handbuch," Vol. 1. 



48 Although normal sera may gradually precipitate on standing, this 

 takes place much more rapidly in precipitin-sera. The spontaneous precipi- 

 tation of normal sera as well as of those under consideration is analogous 

 to what Bechold and others call the "ageing" (altern) of colloidal suspen- 

 sions, which, though originally stable, will eventually settle out, even in the 

 presence of protective colloids. 



