METHODS OF EXAMINATION 



127 



More complete results are available when some method is employed by 

 which the bacteria in a given quantity of air are examined. The oldest 

 method employed, and one which is still used, is that of Hesse. The 

 apparatus is shown in Fig. 50. It consists of a cylindrical tube a about 

 20 inches long and 2 inches in diameter. At one end this is closed by a 

 rubber cork having a piece of quill tubing, /, passing through it and 

 projecting some distance into the interior. For use the tube is sterilised 

 in a tall "Koch," and then a quantity of peptone gelatin, sufficient to 

 cover the whole interior to the thickness of an ordinary gelatin plate, is 

 poured in. This gelatin 



is kept from escaping by ./ ,. 



the projection of the quill 

 tubing into the lumen of 

 the large tube. A plug 

 of cotton wool is now 

 placed in the outer end 

 of the quill tubing. Over 

 the other end of the large 

 tube is tied a sheet of 

 rubber having a hole 

 about a quarter of an inch 

 in diameter in its centre, 

 and over this again is tied 

 a piece of similar but un- 

 perforated sheet rubber. 

 The tube is then steri- 

 lised in the tall " Koch.' 

 On removal from this it 

 is rolled, after the manner 

 of an Esmarch's tube 

 (q.v.), till the gelatin 

 is set as a layer over 

 its interior, and it is then 

 placed horizontally on 

 the tripod as shown. The 

 other part of the appar- 

 atus is an aspirator by 

 means of which a known 

 quantity of air can be 

 brought in contact with 

 the gelatin. It consists 



FIG. 50. Hesse's tube, mounted for use. 



of two conical glass flasks connected by means of a tube which passes 

 through the cork of each down to the bottom of the flask. When this 

 tube is filled with water, it, of course, can act as a syphon tube between 

 volumes of water in the flasks. Such a syphon system being established, 

 the levels of the water are marked on the flasks, and to one a litre of 

 water is added, and by depressing flask b the whole litre can be got into 

 it and the connecting tube c is then clamped. The two flasks are then 

 connected by a rubber tube with the tube/, the clamp on c is opened, 

 and the passing of a litre of water into d will draw a litre of air through 

 the gelatin tube, when the outer rubber sheet is removed from the end 

 and the clamp h opened. By disconnecting at g and reversing the syphon 

 flasks, another litre can be sucked through, and so any desired quantity 

 of air can be brought in contact with the gelatin. The speed ought not 

 to be more than one litre in two minutes, and in such a case practically 



