Petri's Method 



251 



In such a tube, if the air pass through with proper slowness, 

 the colonies will be much more numerous near the point of 

 entrance than near that of exit. The first to fall will prob- 

 ably be those of heaviest specific gravity i. e., the molds. 



Petri's Method. A more exact method is that of Petri, 

 who uses small filters of sand held in place in a wide glass 

 tube by small wire nets (Fig. 

 70). The sand used is made 

 to pass through a sieve whose 

 openings are of known size, 

 is heated to incandescence, 

 then arranged in the tube so 

 that two of the little filters, 

 held in place by their wire- 



Fig. 



70. Petri's sand filter for 

 air-examination. 



Fig. 71. Sedgwick's expanded 

 tube for air-examination. 



gauze coverings, are superimposed. One or both ends of the 

 tube are closed with corks having a narrow glass tube. The 

 apparatus is sterilized by hot air, and is then ready for use. 

 The method of employment is very simple. By means of 

 a hand-pump 100 liters of air are made to jmss through the 

 filter in from ten to twenty minutes, the contained micro- 

 organisms being caught and retained by the sand. The sand 



