STUDIES UPON AGGLUTINATION 67 



c. Most cultures grown at 37°C. for eighteen to twenty-four 

 hours show a slight pellicle on the surface; to avoid mistaking 

 this for spontaneous agglutination the cultures should be shaken 

 and left at room temperature for a while, after the pellicle falls 

 to the bottom of the test tube. 



d. Heavy growths yield better results in the Widal test when 

 diluted 2 or 4 times with salt solution. 



e. Acid broth (0.2 per cent) cultures should be used for routine 

 agglutination tests inasmuch as acidity is useful and greatly 

 helps in weak agglutination. 



/. It is well known that commercial formaUn contains formic 

 acid. Fearing that the formic acid would interfere with the 

 Widal reaction, I have tried certain strains of Bad. typhosum 

 and Bad. paratyphosian group with various dilutions of formic 

 acid (0.1 to 10 per cent) and found that with these dilutions 

 weak chemical agglutinations are produced, but the specific 

 agglutination is diminished. With a weak solution of formalin 

 (0.1 to 1 per cent) no chemical agglutination is produced and 

 spontaneous agglutination is prevented, as above stated. Evi- 

 dently the quantity of formic acid in these concentrations of 

 formalin is so small that it does not interfere with the reaction. 

 Commercial formalin gave satisfactory results throughout this 

 work. 



g. The addition of phenol (0.1 per cent) or corrosive subli- 

 mate (0.01 per cent) did not change normal agglutination power. 

 Tricresol (0.1 per cent) slightly weakened agglutination in the 

 colon-typhoid group. All substances produced weak or nega- 

 tive agglutination reactions when used in more concentrated 

 solutions than those above mentioned. 



This investigation was made possible by a grant from the 

 National Canners xVssociation and has been carried out under 

 direction of Prof. JM. J. Rosenau, of the Department of Preven- 

 tive Medicine and Hygiene, Harvard Medical School, Boston, 

 Mass. 



