REDUCING POWER OF BACTERIA. 233 



the organisms in a litre of culture fluid and then 

 distilling off portions of the culture, collecting in 

 Nessler tubes, and applying 1 c.c. of the reagent to 

 each 50 c.c. of the distillate. The presence of ammo- 

 nia in the distillate is shown by the yellow coloration 

 resulting from the addition of the reagent. 



The direct application of the reagent to the culture 

 will give satisfactory results if a great deal of ammonia 

 has been formed. In this instance the mercury in 

 the reagent will be precipitated as mercurous oxide. 

 Another rough test for the formation of ammonia is to 

 place a strip of filter-paper moistened with the Ness- 

 ler reagent over the mouth of a test-tube containing 

 the culture, and then gently heating the culture. As 

 the ammonia is driven off by the heat, it will react on 

 the reagent on the strip of paper. 



EXAMINATION OF CULTURES FOR BACTERIAL TOX- 

 INS. In the systematic study of a pathogenic organism 

 it is necessary to know whether it is capable of pro- 

 ducing a soluble toxin when growing in culture-media. 

 This is done by filtering cultures of various ages and 

 testing the effect of the filtrate upon susceptible 

 animals. 



FILTRATION OF CULTURES. A variety of filters 

 have been devised for the purpose of filtering liquid 

 cultures and other fluids to obtain sterile filtrates. 

 These filters are usually constructed of unglazed porce- 

 lain or of infusorial earth, and are made in the form of 

 hollow cylinders or bulbs. The best-known forms of 

 bacterial filters are those of Chamberland and of Berke- 

 feld. All the filters used for this purpose require some 

 motive power to force the fluid through the filter. Com- 

 pressed air may be employed to force the fluid through 



