234 ESSENTIALS OF BACTERIOLOGY 



The tube and its contents, the ends having first been 

 plugged with cotton, are sterilized in a hot-air oven at 

 150 C. 



One end of the tube is then fitted with a rubber cork 

 through which passes a glass tube, which is connected with 

 an aspirator (a hand-pump with a known capacity). 



If 100 liters 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, each filter for itself, and 

 thoroughly mixed with gelatin, there should be no colonies 

 developed 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, 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 gage and an air-pump, or ordinary aspirat- 

 ing bottles, the volume 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 gelatin 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 aerobioscope 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, through accident, be lifted 

 into the atmosphere, and under certain conditions may be 

 always present the Bacillus tuberculosis, for example, in 

 rooms where consumptives are living. 



Typhoid fever, influenza, pneumonia, and diphtheria may 

 be conveyed through the air by the cough and expectora- 

 tion of affected persons. 



Non-pathogenic. The micrococci predominate. Sarcinae, 

 yeasts, and molds constantly contaminate cultures. 



