PRECIPITIN TKSTS IX SYPHILIS 451 



Alcohol-Water Ratio — Tlie ratio of water to alcoliol is rela- 

 tively unimportant. Sufficient alcohol must be used to carry in 

 a stable solution the lipoidal substance. The Avater-alcohol ratio 

 must be high enough to prevent solution of the lipoids in the 

 finished suspension. After the suspension is prepared, the alco- 

 hol becomes inert so far as activity of the test is concerned. 



The speed of "shaking" or "'rotation" of the test is important 

 since the velocity of precipitation is chiefly determined by this 

 factor. It is assumed that the increased speed of precipitation 

 is due in part to a more rapid filming of the antigen particles 

 with reagin and partly to the increased number of contacts made 

 between the filmed particles during mixing. The filmed particles 

 have lost their high P.D. and are sticky; hence, once in contact, 

 they tend to precipitate in clumps. The shaking principle w-as 

 introduced into reagin tests by Gaehtgens and Holn and studied 

 in detail by Kahn. According to the latter author, insufficient 

 shaking results in negative reactions from sera low in reagin titer 

 and excessive shaking leads to weak positive reactions in sera con- 

 la ining no reagin. 



Miscellaneous Factors.- — Factors that also affect the size and 

 shape of the dispersed particles are the speed of mixing, the ratio 

 of masses of lipoids and water, the final alcohol or ether content, 

 order of mixing, concentration of electrolytes, temperature of 

 mixing and the age of the emulsion. In the precipitation test, 

 as in the complement fixation test, the state of dispersion of the 

 antigen is an important consideration. 



Finished Antigen. — The finished antigen is a suspension of 

 alcohol-soluble substance precipitated in an aqueous medium. 

 According to Eagle the sensitivity increases, within certain limits, 

 with the coarseness of the dispersed particles. Kline has modified 

 this view, believing that sensitivity increases with size and shape 

 only when all other factors of the test are held constant. Kline 

 considers the shape equally important with surface area. However, 

 on final analysis, as the size of the particle increases, the surface 

 area is decreased per unit volume of suspension, which is the 

 same as reducing the amount of antigen in the test. As the sur- 

 face area decreases, the amount of reagin necessary for precipita- 

 tion is correspondingly reduced. 



