ANALOGY BETWEEN REACTIONS 515 



soluble in an excess of both substances, just as a precipitate is soluble 

 in an excess either of precipitin or of its antigen. 



10. Absorption is the taking up of dissolved or volatile substances 

 by finely divided or colloidal bodies. It is a combination between two 

 substances dependent on physical attraction rather than on chemical 

 affinity, and taking place in variable ratios, rather than in simple and 

 constant ones, as occur in a true chemical union. It is believed by many 

 that the two substances entering into the phenomenon of absorption 

 exist as such side by side in the compound, which is to be regarded 

 as an intimate admixture of the two, rather than as a new compound. 



The lack of definite ratios by which colloids are absorbed has been 

 shown by Bordet in the amount of hemolytic immune body that can be 

 taken up by a given volume of corpuscles i. e., the amount varies 

 according to whether the corpuscles are added at once or in successive 

 small portions. Thus, in one example, 0.4 c.c. of a hemolytic serum 

 dissolved 0.5 c.c. of corpuscles if added at once; but if 0.2 c.c. of cor- 

 puscles was added first and successive amounts of 0.1 c.c. then put in, 

 no solution took place after the one that followed the addition of the first 

 portion. This was explained by Bordet according to the principles of 

 absorption, this observer comparing it with the absorption of a dye by 

 filter-paper. While other explanations are possible, yet exactly analo- 

 gous phenomena may be seen in the mutual absorption of colloids of 

 opposite sign. Thus, as we have previously stated, the addition of 

 a solution of an electropositive colloid to a solution of an electronegative 

 one tends to repel the particles, with the formation of masses for the 

 purpose of self-protection, and in this manner the process of agglutina- 

 tion and precipitation is begun. But if a small amount of a second col- 

 loid is added to the same volume of the others, new aggregates of the two 

 are formed that are less favorable to precipitation and require more of the 

 second colloid to bring about complete precipitation. 



ANALOGY BETWEEN THE REACTIONS OF IMMUNITY AND COLLOIDAL 



CHEMISTRY 



With these few brief remarks on the properties and nature of colloids 

 and the close resemblance of cellular protoplasm and fluids to colloids, 

 we may consider briefly the apparent similarity that exists between the 

 colloidal reactions and some of the reactions of immunity. This is 

 especially pertinent for several reasons: it has been shown that cellular 

 protoplasm is colloidal in nature; that antigens are certainly colloidal, 

 and that antibodies, while they may or may not be solutions of colloids, 



