142 AIR 



expressed per square foot per minute. This simple method 

 gives results nearly as valuable as the more complicated methods, 

 and in the bacteriological examination of air should never be 

 neglected. 



Additional results of value can be obtained by using Petri- 

 dishes containing media other than nutrient gelatine or agar. 

 For example, by using bile salt neutral red lactose agar or 

 fuchsin agar, valuable data as to the relative number of B. coli 

 and allied organisms may be obtained. 



2. Trapping the Organisms by Means of Filters of Solid 

 Material In the earlier work in this direction insoluble filters 

 were used. Thus Pasteur used an asbestos filter and Petri one 

 of sand. 



Petri's sand-filter is convenient, but it has the objection that 

 the particles of sand may be mistaken for colonies. The sand, 

 after trapping the organisms in the same way as for the sugar- 

 filters described below, is mixed with liquefied gelatine, which is 

 distributed in Petri-dishes, solidified, incubated, and counted. 



At the present day soluble filters are often used, cane-sugar 

 being the most satisfactory material. 



Of apparatus of this kind the Sedgwick-Tncker tube is a con- 

 venient form. . The tube used consists of a glass cylinder about 

 12 inches long, part being a wide cylinder, bore about ij inches, 

 and the rest a fine glass tube, bore about J inch, as shown in 

 Fig. 15. 



The free end of the narrow tube is plugged with cotton-wool; 

 above this is placed a coil of wire gauze, and above this and 

 supported by it is the sugar, which occupies the rest of the tube. 

 The wide free end of the cylinder is partially constricted and 

 plugged with cotton-wool, the cylinder itself being empty. The 

 whole is then sterilized in the hot-air sterilizer for three hours at 

 120 C. It is an advantage, however, to sterilize before the sugar 

 is put in, at a higher temperature e.g., 15 to i6oC. ; then 

 put in the previously dried sugar, and sterilize again for three 

 hours at 120 C. 



The apparatus is fixed horizontally in the air which is to 

 be examined. To draw the air through, the narrow tube is 



