AIR, SOIL, AND WATER. 



183 



If a hundred litres of air pass through the tube in fifteen min- 

 utes the germs should all be arrested in the first sand filter. 



And when the filters are removed and thoroughly mixed with 

 gelatine, each filter for itself, there should be no colonies de- 

 veloped from the second filter, i. e., the one nearest the aspirator. 



Sedgwick-Tucker Method. A special form of tube is used, 

 called an aerobioscope. It consists of a neck 2.5 cm. in length, 



FIG. 95. 



Sedgwick-Tucker aerobioscope. 



an expanded portion 15 cm. long, and a long narrow tube of 15 

 cm. After sterilization the tube is partly filled with granulated 

 sugar, which is the filtering material. By means of a vacuum 

 gauge and an air-pump, or ordinary aspirating bottles, the vol- 

 ume of air passing through the apparatus can be determined. 

 After the air has been passed through, the sugar is gently 

 shaken from the narrow tube into the expanded portion, and 

 20 c. c. of liquefied gelatine is poured in. The sugar dissolves, 

 and the mixture is then rolled on the inner side of the glass 

 as an Esmarch tube. This part of the apparatus is divided into 

 squares to make the counting of colonies easy. The aerobio- 

 scope is very highly recommended. 



Varieties Found in Air. The only pathogenic bacteria found 

 with any constancy are the staphylococcus aureus and citreus; 

 but any bacterium can be, through accident, lifted into the atmo- 

 sphere, and in certain places may be always found the bacillus 

 tuberculosis, for example, in rooms where many consumptives 

 are living. 



Non-Pathogenic. The micrococci predominate. Sarcinse, 

 yeasts, and moulds constantly contaminate cultures. 



In the ordinary habitations the average number of germs to 

 the litre of air does not exceed five. 



Around water-closets, where one would imagine a great num- 

 ber to exist, owing to the undisturbed condition of the air, but 

 few will be found. 



