THE EXAMINATION OF MILK 429 



bottom. The deposit is separated from the fluid by means 

 of a fairly well-fitting ball, which is let into the tube on a 

 stalk. The bottom of the tube is then scraped, the deposit 

 divided between two cover-glasses and stained for the 

 tubercle bacillus. 



Undoubtedly the most satisfactory method of examining 

 milk for the tubercle bacillus, if time is no object, is by the 

 intraperitoneal or subcutaneous injection into guinea-pigs. 



Examination for the Typhoid Bacillus. — The milk can be 

 examined by one of the methods described under 'The 

 Examination of Water.' 



Determination of the Number of Organisms. — The numerical 

 determination of the bacteria present in milk can be made 

 by the method already described under ' The Examina- 

 tion of Water,' except that the dilution, owing to the much 

 larger number of organisms present, requires to be carried 

 to a much greater extent. If the samples cannot, be 

 plate-cultured at once, they should be allowed to remain 

 in an ice-safe, otherwise the results, owing to the rapid 

 multiplication of the bacteria, will have but little practical 

 value. 



THE BACTERIOLOGICAL EXAMINATION OF AIR. 



The air does not normally contain any characteristic 

 bacterial flora, as the organic matter required for their 

 growth is not found in the air to any considerable extent. 

 Their presence is due to the fact that they are blown about 

 with the dust by air currents and winds from surfaces 

 where they exist in a dried-up condition. Bacteria do not 

 themselves unaided rise into the air ; when air-currents are 

 absent they always sink under the influence of gravity to 

 the ground, where they always find better conditions for 

 their growth and development. Wherever the greatest 



