866 THE BACTERIOLOGICAL EXAMINATION OF AIR 



When the desired volume of air has been aspirated the powdered sodium 

 sulphate is dissolved in a known volume of broth and plates are sown with 

 measured quantities of the liquid. As a control the asbestos plug is trans- 

 ferred with sterile forceps to a tube of broth and this of 

 course should remain sterile. 



[III. Andrewes' method. The filtering medium consists 

 of a mixture of glass wool (3-4 parts) and cane sugar 

 (1 part). A straight piece of glass-tubing is used without 

 any constriction and the medium rammed in fairly tightly. 

 After plugging the ends with wool the apparatus is steril- 

 ized at 120 C. and after aspirating the air through it the 

 mixture is pushed out with a sterile glass rod into a plate 

 of melted sterile gelatin.] 



B. Methods based upon bubbling the air through 



liquids. 



I. Method of Straus and Wurtz. The apparatus consists 

 of a glass cylinder with an appendix at its lower end filled 

 with 10 c.c. of liquefied gelatin the surface of which is 

 covered with a few drops of oil. 



The upper part of the cylinder is furnished (1) with a 

 lateral tubulure plugged with wool and (2) with a central 

 J ground glass opening which is hermetically closed with a 



"^ glass tube reaching below to the bottom of the gelatin 



in the appendage and above projecting beyond the cylinder 

 which is plugged with wool. The apparatus is sterilized 

 in the autoclave. When required for use, the lower part 

 of the cylinder is placed in water at about 10 0. to liquefy 

 the gelatin, the lateral tubulure is attached to an aspirator 



and the wool plug removed. The as- 

 pirated air passes through the central 



tube B and bubbles through the gelatin 



in which it deposits the organisms sus- 

 pended in it. (The layer of oil prevents 



the gelatin frothing.) When 10 litres of 



air have been aspirated the aspirator is 



disconnected, and air is gently blown in 



through the lateral tubulure, thus driving 



the gelatin up into the central tube. This 



operation is repeated several times in order 



to thoroughly wash the tube. Finally, 



plates are poured with the gelatin. [The 



apparatus can be used for the simple 



enumeration of organisms, the gelatin in 



this case being run over the sides of the 



cylinder after the fashion of an Esmarch's 



roll tube.] 



This apparatus is very convenient but many organisms are arrested in the delivery 

 tube which is very long and has an irregular surface, so that the results are not very 

 accurate ; moreover the apparatus is only available for small volumes of air. 



II. Miguel's method. The apparatus consists of a Pasteur flask with two 

 lateral tubulures attached to opposite sides of its upper part and with a 

 central tube dipping to the bottom. A ground glass cap closes the central 

 tube : one of the lateral tubulures is plugged with wool and the other, used 



FIG. 414. Straus and 

 Wurtz' apparatus. 



FIG. 415. Miquel's apparatus. 



