BACTERIOLOGICAL EXAMINATION OF AIR 75 



this process are that dried gelatine does not catch germs like the broth cultures 

 of Pasteur or Miquel, and that many organisms are carried straight through the 

 tube, and failing to be deposited, pass out at the aspirator exit, and thus are 

 neither caught nor counted. The Hesse tube is generally used in practice with a 

 pump consisting of two flasks and a double-way indiarubber tube. The flasks have 

 a capacity for one litre of water. By a simple arrangement it is possible to secure 

 syphon action, and hence measure with considerable exactitude the amount of air 

 passing through the tube (Plate 5). 



4. Methods of Filtration.— YvaxManA, 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 different 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. Probably it was this which suggested 

 the use of soluble filters like sugar. Pasteur introduced the principle, and Frank- 

 land and others have followed it out. Sedgwick's Tube consists of a comparatively 

 small glass tube, about a foot long. Half 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. After sterilisation an indiarubber 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 matter 



l( 



^m^^sL 



\ r 



Pig. 12.— Sedgwick's Sugar-tube. 



is caught in the sugar. Warm, nutrient gelatine (10-15 c.c.) is now poured into the 

 broad end of the tube, and by means of a sterilised stilette the sugar is pushed down 

 into the gelatine, where it quickly dissolves. We have now in the gelatine all the 

 micro-organisms in the air which has been drawn through the tube. After plugging 

 with wool at both ends, the tube is rolled on ice, or under a cold-water tap, in order 

 to fix the gelatine all round the inner wall of the tube, which is incubated at room 

 temperature. In a day or two the colonies appear, and may be examined. 



Frankland used finely powdered sugar and glass wool as filtering-medium, and 

 a tube with two constrictions. After passing sufScient air through, the tube is 

 broken in halves and the wool and sugar are pushed by means of a sterile needle 

 into liquefied gelatine. The sugar dissolves and the organisms are distributed in 

 the medium. Andrewes has used a modification of this method, and the aspiration 

 was carried out with a large brass syringe of known capacity, fitted with a two-way 

 nozzle and cock, so that the requisite number of syringefuls could be aspirated 

 without disturbance.* 



Various other methods, including Miquel's filtration method, and the methods 

 of Laveran, and Wiirtz and Strauss, have been used, but the principal are those 

 mentioned above. 



In respect of the results obtained in the examination of air bacteriologically, it 

 may be said that they are twofold. First, a quantitative result is obtained by 

 which we may arrive at the approximate number of bacteria and moulds. Secondly, 

 the quality or species of organisms is determined. Reference will be made to both 

 these points in the pages which follow. 



* Brit. Med. .Tour., 1902, ii., p. 1534; and Report to London County Counril, 

 1902. 



