AIR 



143 



connected by means of india-rubber tubing to an aspirator. 

 Just before use the cotton-wool plug in the large 

 end is scorched in the flame of a spirit lamp and 

 removed. Air is then drawn through by means of 

 the aspirator. At least 40 litres of air should be 

 aspirated through. The amount of air examined is, 

 of course, known by the amount of water let out 

 from the aspirator. When the air has been drawn 

 through, the cotton-wool plug, well burned in the 

 flame, is replaced. The rest of the examination is 

 done in the laboratory. A tubeful of liquefied 

 gelatine (about 10 to 15 c.c.) is introduced into the 

 cylindrical part, the cotton-wool plug being momen- 

 tarily removed for the purpose, and by means of a 

 sterilized rod the sugar is pushed out into the gelatine 

 in which it dissolves. After thorough mixture a 

 roll-tube is made in the ordinary way. The whole 

 apparatus is then incubated in the cool incubator, 

 and the colonies which develop are counted and, if 

 necessary, subcultivated. 



This apparatus has all the disadvantages of a 

 roll-tube i.e. y it is difficult to examine the colonies 

 microscopically, and liquefaction of some colonies 

 may contaminate and spoil others, while only those 

 organisms which can develop at gelatine temperatures 

 will grow. Fig I5> 



Sedgwick- 



FranklancCs Method. In this method special tubes, tube. 

 5 inches long and with a diameter of \ inch, are used. 

 Their form and the arrangement of plugs are sufficiently shown 

 in the figure (Fig. 16). 



The tubes are fitted up and each placed in a separate outer 

 tube closed by an asbestos plug (as recommended by Haldane). 

 They are then sterilized by dry heat for three hours at 1.30 C. 



When required for use, the tube is quickly removed from its 

 outer case, connected to the aspirator by the plugged (c) end 

 and placed in position. The plug a is then withdrawn, and the 

 aspirator set in action. When the requisite volume of air has 



