BACTERIA IN THE AIR 99 



hence measure with considerable exactitude the amount of 

 air passing through the tube. 



5. Methods of Filtration. To-day most of the above 

 methods have been discarded, with the exception, perhaps, 

 of Miquel's and modifications thereof. 



Frankland, Petri, Pasteur, Sedgwick, and others have 

 suggested the adoption of methods of filtration. These 

 depend upon catching the organisms contained in the air by 

 filtering them through sterilised sand or sugar, and then 

 examining these media in the ordinary way. Many differ- 

 ent kinds of apparatus have been invented. Petri aspirates 

 through a glass tube containing sterilised sand, which after 

 use is distributed in Petri dishes and covered with gelatine. 

 The principal objection to this method is the presence of the 

 opaque particles of sand in and under the gelatine. Prob- 

 ably it was this which suggested the use of soluble filters 

 like sugar. Pasteur introduced the principle, and Frankland 

 and others have followed it out. The apparatus most largely 

 used is that known as Sedgwick's Tube. This consists of 

 a comparatively small glass tube, about a foot long. Half 



J 



SEDGWICK'S SUGAR-TUBE 



of it has a bore of 2.5 cm., and the other half a bore of .5 

 cm. It is sterilised at 150 C, after which the dry, finely 

 granulated cane-sugar is inserted in such a way as to occupy 

 an inch or more of the narrow part of the tube next the 

 wide part. Next to it is placed a wool plug, and the whole 

 is again sterilised at 130 C. for two hours, care being taken 

 that the sugar does not melt. After sterilisation an india- 

 rubber tube is fixed to the end of the narrow portion, and 

 thus it is attached to the aspirator. The measured quantity 

 (5-20 litres) of air is drawn through, and any particulate 



