NUTRIENT JELLY— EXAMINATION OF AIR. 89 



the cavity of the flask. Then the pipette is pushed down into the 

 liquefied gelatine, 10 c.c. of which are drawn up into the bulb by 

 suction. The upper end of the pipette is closed by the thumb, and the 

 pipette withdrawn. During the withdrawal of the pipette the cotton 

 wool plug is pressed firmly against it, to prevent ingress of dust par- 

 ticles ; and so soon as the point of the pipette is disengaged from the 

 cotton wool plug, that is replaced in its proper position. ] The same 

 manoeuvres are carried out in introducing the pipette into the sterile 

 jar; and when this is done, the liquid jelly is allowed to trickle 

 down into the capsule, and the pipette removed with the same pre- 

 cautions as before. We have thus obtained a thin layer of a very 

 susceptible substance, which may be exposed to any atmosphere ; 

 and the reaction that occurs can be easily observed through the walls 

 of the glass jar without in any way disturbing the capsule in which it 

 takes place. The jelly soon solidifies ; and after being kept for a 



Fig. 46. — From a photograph of glass capsule exposed in Saint 

 Giles' Cathedral, Edinburgh (see description). 



week in a warm room, to make sure that no adventitious growth will 

 take place in the capsule, it may be exposed to any given atmosphere 

 by the removal of the cotton wool plug. This exposure is of a definite 



1 This method is open to the objection that it cannot be repeated safely more 

 than once or twice, as the risks of contamination are greatly increased by the 

 presence in the plug of even a small quantity of the nutrient material ; a better 

 plan is to remove the plugs from both beaker and flask, and carry out all the 

 manipulations under the protection of the carbolic spray. 



