148 



BACTERIA IN AIR 



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 unperforated sheet rubber. The tube is then sterilised 



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 appa- 

 ratus is an aspirator, by 

 means of which a known 

 quantity of air can be 

 brought in contact with 

 the gelatin. It consists 

 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 es- 

 tablished, the levels of 

 the water are marked on 

 the flasks, and to one a 

 litre of water is added ; 

 by depressing flask b the 

 whole litre can be got 

 into it, and the connect- 

 ing tube c is then clamped. The two flasks are now 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 all the 

 organisms will be found to have fallen out of the air on to the gelatin 

 in the course of their transit. This fact can be tested by interposing 

 between the tube a and the aspirator a second tube prepared in the same 

 way, which ought, of course, to show no growth. When forty-eight 

 hours at 20 C. or four days at lower temperature have elapsed, the 



FIG. 48. Hesse's tube, mounted for use. 



