EXAMINATION OF AIR 605 



that particles of saliva are disseminated as far as 40 feet 

 in the act of loud speaking, indicating the possibility of 

 the wide distribution of such pathogenic organisms as 

 the tubercle, plague, and influenza bacilli and the pneumo- 

 coccus by speaking, and still more so by coughing. 



The number of dust particles in the air may be enormous. 

 In London Macfadyen and Lunt observed as extremes from 

 20,000 to nearly 600,000 per c.c. The ratio of micro- 

 organisms to dust particles is therefore a very small one. 



Bacteriological Examination of Air 



A number of methods have been devised for the estimation of 

 the number of micro-organisms in the air, of which the following 

 are the principal ones : 



(1) Plate method. Melted sterile nutrient gelatin is poured into 

 a sterilised Petri dish, and allowed to set. The plate is then exposed 

 to the air, by removing the lid, for a given time one, five, ten, 

 or fifteen minutes, etc. the lid is replaced, and the plate incubated 

 at 22 C. for some days. The number of colonies of moulds, bacteria, 

 yeasts, etc., is counted, and, having estimated the area of the 

 gelatin plate, 1 the result is expressed as the number of organisms 

 falling per square foot per minute. The results obtained by this 

 method are roughly comparative, but no estimate can be formed 

 from it of the number of organisms contained in a given volume of 

 the air. 



(2) Hesse's method. This is a quantitative method for estimating 

 the number of organisms contained in a given volume of air. The 

 apparatus consists of a glass tube 30 in. long by 1^ to 2 in. in diameter. 

 One end of this tube is plugged with a rubber cork through which 

 a glass tube passes, the other end is covered with a piece of sheet 

 rubber perforated with a hole to | in. in diameter ; over this is 

 placed another sheet of rubber, unperforated. The small tube 

 being plugged with cotton-wool, the whole is sterilised for an hour 

 in the steam steriliser. Just before use 40 to 50 c.c. of melted 

 sterile nutrient gelatin are poured into the tube, and its walls 

 coated with the medium. The tube is then strapped horizontally 

 on to a tripod stand, and the small tube connected by means of a 



1 The area of a circular dish is calculated by multiplying the square 

 of the diameter by 0'785. 



