124 



BACTERIA IN AIR, SOIL, AND WATER. 



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 apparatus 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 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 i 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 colonies which develop in a may be counted. The disadvantage 

 of the method is that if particles of dust carrying more than one bacterium 

 alight on the gelatin, these bacteria develop in one colony, and thus the enu- 

 meration results may be too low; difficulties may also arise from liquefying 

 colonies developing in the upper parts of the tube and running over the 

 gelatin. 



FIG. 57. Hesse's tube, mounted for use. 



