Sedgwick's Method 235 



of gelatin in the same manner as an Esmarch tube. The tube is closed at 

 both ends with sterile corks carrying small glass tubes plugged with cotton. 

 When ready for use the tube at one end is attached to a hand-pump, the cotton 

 removed from the other end, and the air slowly passed through, the bacteria hav- 

 ing time to sediment upon the gelatin as they pass. When the required amount 

 has passed, the tubes are again plugged, the apparatus stood away for a time, 

 and subsequently, when they have grown, the colonies are counted. The 

 number of colonies in the tube will represent pretty accurately the number of 

 bacteria in the volume of air that passed through the tube. 



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 probably 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. 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-gauze coverings, are superimposed. One or both ends 



Fig. 80. Hesse's apparatus for collecting bacteria from the air. 



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 pass 

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

 being caught and retained by the sand. The sand from the upper filter is then 

 carefully mixed with sterile melted gelatin and poured into sterile Petri dishes, 

 where the colonies develop and can be counted. Petri points out in relation to 

 his method that the filter catches a relatively greater number of bacteria in 

 proportion to molds than the Hesse apparatus, which depends upon sedimenta- 

 tion. Sternberg points out that the chief objection to the method is the presence 

 of the sand, which interferes with the recognition and counting of the colonies 

 in the gelatin. 



Sedgwick's Method. Sedgwick and Miquel have recommended the use of 

 a soluble material granulated or pulverized sugar instead of the sand. The 

 apparatus used for the sugar experiments differs a little from the original of Petri, 

 though the principle is the same, and can be modified to suit the experimenter. 



A particularly useful form of apparatus, suggested by Sedgwick and Tucker, 

 has an expansion above the filter, so that as soon as the sugar is dissolved in the 



