SOLID TRANSPARENT MEDIA 55 



The addition of the white of an egg, as before mentioned, 

 will often clear it up ; if this avails not, refiltering several times 

 and attention to the few points mentioned will produce a clear 

 solution. 



Sterilizing the Gelatin. The gelatin is kept in little flasks 

 or poured at once into sterile test-tubes, careful not to wet the 

 neck where the cotton enters, lest when cool the cotton plug 

 stick to the tube. 



The tubes are then placed in steam-chest for three successive 

 days, fifteen minutes each day (or in water-bath one hour a 

 day for three days). Then set aside in a temperature of 15 

 to 20 C., and if no germs develop and the gelatin remains 

 clear, it can be used for cultivation purposes. 



Modification. The amount of gelatin added to the meat- 

 water can be variously altered, and instead of making gelatin 

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

 Glycerin (4 to 6 per cent.) 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 gelatin, retaining its solidity at a 

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

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



Agar 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 grams of agar to 1000 

 grams of 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 

 alkaii 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 



