138 VERMONT AGRICULTURAL REPORT. 



washed once or twice with a small amount of water. The fil- 

 trate, including the washings, is then evaporated, and may be 

 concentrated to any desired volume. As a rule, one-fifth of the 

 quantity of the original culture is the most convenient point to 

 be reached in the evaporation. Instead of filtering off the 

 germs and then evaporating the filtrate, the entire contents of 

 the flask may be concentrated by evaporation and the solution 

 filtered after it has been concentrated. The results are the same 

 but the writer's preference is for the first method. 



DILUTION OP TUBERCULIN. 



Instead of sending out concentrated tuberculin, it has been 

 found advisable to dilute it to such a strength that 2 cubic cen- 

 timeters will be a suitable dose for the purpose of diagnosing 

 tuberculosis in cattle. To accomplish this dilution, there is 

 added to the concentrated tuberculin glycerin equal to one- 

 fourth the original bulk of the cultured liquid. This is then diluted 

 with one-fourth of 1 per cent. carbolic acid, so that the volume of 

 the tuberculin usually obtained is one- fourth more than the orig- 

 inal quantity of culture media used. In other words, 1,000 cubic 

 centimeters of culture media, after the germ has been allowed 

 to grow sufficiently long, diluted with the proper amount of 

 glycerin and carbolic acid, should give 1,250 cubic centimeters 

 of tuberculin of such a strength that 2 cubic centimeters would 

 be a satisfactory dose for testing an animal of 1,000 pounds in 

 weight. 



METHODS OF STANDARDIZING TUBERCULIN. 



Several methods of standardization of the tuberculin are 

 adopted. The amount of the tuberculin required to produce a 

 rise of 4 or 5 in temperature in tuberculous guinea pigs of 1 

 pound in weight is noted, or the quantity of tuberculin found 

 necessary to kill a tuberculous guinea pig ; or, again, a standard 

 upon a chemical basis, depending upon the amount of acid 

 which is produced by the growth of the bacilli, if used. All 

 tubercle cultures after they are well grown show a decided acid 

 reaction. If the reaction of the media is carefully noted before 

 inoculation, and again after the growth of the germ, it is found 

 that the amount of acid reaction is increased proportionately to 

 the quantity of the growth. By practice, it is also found that this 

 is approximately constant. If the value of a tuberculin made 

 from a culture in which the acid reaction has been carefully 

 tested is once noted it serves as a guide for future work. 



In practice, it has been found that healthy animals do not 

 give reactions with large doses of tuberculin, and that as a rule 



