88 



SOLID CULTIVATION MEDIA. 



outcome of the growth of a single spore, that has found its way into 

 the jelly, in all probability by falling upon the inoculating wire whilst 

 it was passing through the air to the sterile test-tube. 



(B.) In Capsules, for testing Air. 



45. The condition of the atmosphere, in respect to the number of 

 germs in it, may be ascertained approximately by the employment of 

 a cultivation experiment with sterile jelly. A glass jar plugged with 

 cotton wool, and containing a shallow glass capsule (Fig. 45), is 



Fig. 45. — Apparatus for determining relative purity of atmo- 

 spheres (qualitative). 



a. Capsule containing sterilised nutrient jelly. 



b. Brass lifter for removing capsule for examination. 



c. Glass jar with sterilised plug, containing lifter and capsule ready 



for use. 



sterilised by heating to 170° C. dry heat for one hour. When it has 

 cooled, the glass capsule is charged with 10 c.c. of jelly, which has 

 been previously liquefied by heat, and withdrawn from one of the 

 stock-flasks by means of a sterilised (superheated) glass pipette. 

 This has to be done with great care — firstly, to avoid the risk of con- 

 taminating the contents of the stock-flask ; and, secondly, to prevent 

 the introduction of dust particles along with the jelly into the sterile 

 jar prepared for the experiment. The cotton wool plug of the 

 stock-flask is half withdrawn, and the point of the pipette is slowly 

 thrust through the remaining layers of the plug till it is free within 



