216 AIDS TO BACTERIOLOGY 



70 cm. by 3-5 cm., covered at one end by two rubber 

 caps, the inner one having a hole in its centre 10 mm. 

 in diameter, and at the other end a rubber cork fits in the 

 cylinder. Through this cork a glass tube 100 mm. in 

 length passes, which is plugged with cotton-wool. The 

 cylinder is sterilised for one hour in the steam steriliser. 

 The rubber stopper is removed, and 50 c.c. of nutrient 

 gelatin in a fluid condition is introduced into the tube and 

 rolled out on the sides, as in the preparation of an Esmarch's 

 tube, leaving a somewhat thicker coating along the under 

 side of the cylinder. The cylinder and its fittings are 

 mounted on a tripod stand, and the glass tube which passes 

 through the rubber stopper is connected by means of a 

 rubber tube with an aspirator, the cotton having first 

 been removed from its outer end. The aspirator most 

 suitable for the purpose is the double wash-bottle arrange- 

 ment, which is conveniently attached to the stand by means 

 of hooks. The outer rubber cap is then removed and the 

 aspirator started. Air is drawn through the tube by 

 suction, the micro-organisms contained therein falling 

 on the gelatin. The amount of air entering is estimated 

 by the capacity of the flasks forming the aspirator. The 

 rate at which it enters is controlled by the flow of the water, 

 which can be regulated by a pinchcock. Hesse advises 

 that the amount and the rate of flow for rooms and closed 

 spaces should be about 1 to 5 litres, passed at the rate of 

 \ litre a minute. For open spaces 10 to 20 litres is passed 

 at about four minutes per litre. The tube is then capped 

 and the colonies allowed to develop, after which they can 

 be further examined by subcultures. 



Petri's Method. The air is led through glass tubes 

 packed with fine sand, kept in place by plugs of glass- 

 wool, the organisms being retained by the sand filter, and 

 afterwards the sand is pushed out of the tube with a 

 sterile wire and plated out in gelatin. In this method the 

 sand, being insoluble, is troublesome, interfering with a 

 clear view of the colonies. 



FranklaruTs Method. Except for the substitution of 

 powdered cane-sugar for the sand, the process is the 

 same as that of Petri. 



Sedgwick and Tucker's Method. A special tube, re- 

 sembling a narrow cylindrical separating funnel, without 

 a stopcock, is used. Powdered cane-sugar is packed into 



