54 ESSENTIALS OF BACTERIOLOGY. 



days, 15 minutes each day (or in water-bath 1 hour a day for 

 three days). Then set aside in a temperature of 15 to 20 C., 

 and if no germs develop and the gelatine remains clear, it can 

 be used for cultivation purposes. 



Modifications. The amount of gelatine added to the meat- 

 water can be variously altered, and instead of making gelatine 

 bouillon milk, blood, serum, urine, and agar can be added. 

 Glycerine (4 to Q%>) is a common addition, and sometimes 

 reducing agents to absorb the oxygen are mixed with it. 



Agar-Agar. This agent, which is of vegetable origin, derived 

 from sea-plants gathered on the coasts of India and Japan, has 

 many of the properties of gelatine, retaining its solidity at a 

 much higher temperature; it becomes liquid at 90 C. and con- 

 geals again at 45 C. Gelatine will liquefy at 35 C. 



It is not affected very much by the peptonizing action of 

 the bacteria 38 C. is the temperature at which most patho- 

 genic germs grow best. 



Preparation of Agar-Agar Bouillon or Nutrient Agar. The 

 ordinary bouillon is first made, and then the agar cut in small 

 pieces, added to the bouillon (15 grammes of agar to 1000 

 grammes bouillon. It is allowed to stand several minutes until 

 the agar swells, and then placed in water-bath or steam-chest 

 for six hours or more. It is then neutralized, very little of the 

 alkali being sufficient. 



A white of an egg added, and boiled for several hours longer, 

 when, even if not perfectly clear, it is filtered. 



The filtering process, very difficult because of the readiness 

 with which the agar solidifies, must be done in steam-chest or 

 with hot-water filter, and very small quantities passed through 

 at a time, changing the filter-paper often. 



Cotton can be used instead of filter-paper, or filtering entirely 

 dispensed with, simply decanting. 



A.S agar is seldom clear, a little more or less opaqueness will 

 not harm. The test-tubes are filled as with the gelatine, and 

 sterilized in the same manner. While cooling, some of the 

 tubes can be placed in a slanting position, so as to obtain a larger 

 surface to work upon. 



Water of condensation will usually separate and settle at the 



