AGGLUTININ UNIT. 217 



are permanently agglutinated are considered in- 

 dicative of a positive reaction; the test is most 

 decisive when large masses are formed, so large that 

 they are seen readily with a low magnification. A 

 similar preparation to which no serum has been 

 added should always be made, in order to eliminate 

 spontaneous or "auto-agglutination" as a possible 

 source of error. 



In a macroscopic test, the uniform cloudiness of The Macro- 

 the mixture of serum and bacteria becomes 

 changed by the formation of smaller and larger 

 flakes or clumps of bacteria, which in the course of 

 a few hours sink to the bottom as a white precipi- 

 tate, leaving a clear overlying fluid. Here also a 

 control tube, to which no serum has been added, 

 should be preserved for comparison. 



The body temperature, which may be obtained 

 in a thermostat, facilitates the reaction. 



The value of an agglutinating serum can not be Tlie 

 expressed in units with the exactness that is at- nln TJnlt 

 tained in measuring diphtheria antitoxin for the 

 following reasons : 1, The limits of the reaction are 

 not sufficiently definite ; 2, a given mass of bacteria 

 has the power of absorbing varying amounts of 

 the agglutinating substance, depending on the con- 

 centration of the latter; and 3, it is impossible to 

 obtain standard bacterial emulsions. 



One may arbitrarily decide on a unit similar to 

 that of Ziipnik, in which a serum which is able 

 to agglutinate a given mass of bacteria in a dilu- 

 tion of one in forty is taken as the standard. If a 

 similar amount of a serum agglutinates in a di- 

 lution of 1 in 120 it is said to be of threefold 

 strength. 



