BACTERIOLOGY OF AIR 729 



the walls of which were smeared with glycerin, Tyndall 

 was able to free it completely from particles and organ- 

 isms. The writer found from 43 to 150 organisms per 

 10 litres of air in some of the principal streets of London 

 during the daytime. 



Gordon, 1 by exposing dishes of neutral-red broth to the 

 air, or by aspirating air through neutral-red broth, has 

 been able to detect the presence of the S. salivarius, 

 M. epidermidis, and scurf micrococcus in air subjected 

 to human contamination. By these tests and by the 

 use of B. prodigiosus as an indicator he concludes 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, 2 the result is expressed as the number 

 of organisms falling per square foot per minute. The results 



1 Reps. Ned. Off. Loc. Gov. Board for 1902-1904. 



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

 diameter by 0-785. 



