76 OUTLINES OF BACTEEIOLOGY 



larger than this enters the body every time half a pint of fresh 

 milk is swallowed. 1 



The following statistics give a very fair idea of the distribution of 

 bacteria in various places : 



No. PER Cub. Metre. 



Mont Souris Park (Paris), 455 



Middle of Paris, 4,000 



Tailors' Room, Whitechapel, 17,000 



Capmakers' Room, 9,000 



Boot Workshop, 25,000 



Railway Works, Wilts, 20,000 



Chocolate Factory, 8,000 



Printing Shop, 9,000 



Ropemakers' Shop, 20,000 



Terrace of House of Commons, 4,200 



The bacteriological examination of the atmosphere is best carried 

 out by means of the Sedgwick sugar tube. This tube is about a 

 foot long, of which half has a bore of 2-5 cm., while the other 

 half has one of '5 cm. After sterilisation an inch or more of the 

 narrow part is filled with sterilised granulated cane sugar. A sterilised 

 india-rubber tube is then attached by one end to the narrow part, and 

 by the other end to an aspirator. A measured quantity of air is drawn 

 through (about 10 litres), and of course all the bacteria are caught in 

 the sugar. Then warm nutrient gelatine is poured into the broad 

 part of the tube, and into this the sugar is thrust by means of a 

 sterilised glass. After the sugar has melted the tube is turned round 

 so as to spread the nutrient gelatine over as large a surface as possible 

 on the glass. When it has solidified the tube is set aside to let each 

 microbe that was inhaled by the tube grow into a colony. By counting 

 the colonies we can estimate the number of bacteria that have been 

 inhaled. As we know the volume of air passed through the sugar we 

 can estimate the number of bacteria per unit volume. 



Conditions determining the Bacterial Contamination of Air. Speaking 

 generally, we may say that the more dust in the atmosphere the greater 

 the number of contained microbes, because dust particles are usually 

 covered with them. That such is the case can be proved by making a 

 bacteriological examination of the air in a room, first when the dust is 

 undisturbed, and then after it has been raised, when a considerable 



^ Lest a false inference be made from this assertion it is well to observe that 

 whereas the swallowed germs enter the stomach, those that are inhaled enter the 

 lungs and air passages which do not oflfer anything like the same resistance to 

 the attacks of pathogenic bacteria. 



