DISEASE, INFECTION, AND RESISTANCE 289 



intervals, and after a time the rabbit bled, the blood allowed 

 to coagulate and the serum secured, this serum, when added 

 to a dilute solution of egg white will cause the medium to 

 become opaque or cloudy and a floeculent precipitate will 

 settle out. This will occur, however, only when the blood 

 serum is mixed with the particular protein injected in the 

 first instance. The substance produced in the blood serum 

 is termed a precipitin. 



Colloidal Solutions and Suspensions. — It was not at first 

 recognized that the phenomena of agglutination and pre- 

 cipitation were fundamentally the same. Substances in 

 extreme state of division or fineness are comparatively 

 coarse colloids, or rather, their suspension constitutes a 

 colloidal suspension. The protein molecule, such as that 

 of egg white, apparently is large enough, or exists in aggre- 

 gates which are large enough, to behave as suspended par- 

 ticles rather than as substances in true solution. Experi- 

 ence has shown that a great variety of substances added to 

 particular proteins will cause precipitation, as in the so- 

 called salting out process. A saturated solution, for 

 example, of ammonium sulphate added to many proteins 

 will cause them to go out of solution, or precipitate. The 

 same is true with bacterial suspensions. A saturated solu- 

 tion of ammonium sulphate will also cause typhoid bacilli 

 to fiocculate out of suspension. This ability of certain salts 

 to cause flocculation or agglutination is particularly well 

 shown by mixing fine silt or clay with water. The solution 

 will remain turbid almost indefinitely, that is, the particles 

 will settle out very slowly indeed unless some salt, such as 

 alum be added. When this is added, however, the clay 

 particles bunch together, flocculate and settle to the bottom 

 rapidly. 



The resemblance between the phenomena of agglutination 

 and precipitation by means of blood serum and the agglu- 

 tination or flocculation by means of salts escaped notice for 



