1 40 PA THOGENIC BA CTERIA . 



be examined pass through a horizontal sterile tube about 

 70 cm. long and 3.5 cm. wide (Fig. 37), the interior of 

 which is coated with gelatin in the same manner as an 

 Esmarch tube. The tube, having been prepared, is 

 closed at both ends with sterile corks carrying smaller 

 glass tubes closed with cotton. When ready for use the 

 tube at one end is attached to a hand-pump, the cotton 

 is removed from the other end, and the air passed through 

 very slowly, the bacteria having time to precipitate 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 colo- 

 nies in the tube will represent pretty accurately the 

 number of bacteria in the amount of air which 

 passed through the tube. 



In such a cylindrical culture it will be noted 

 that if the air is passed through with the 

 proper slowness, the colonies will be much 

 more numerous near the end of entrance than 

 that of exit. The first to fall will probably 

 be those of heaviest specific gravity i. e. the 

 moulds and yeasts. 



A still 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. 38). 

 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 

 FIG. 38. superimposed. One or both ends of the tube 

 etn s san ^^ closed with corks having- a narrow glass 



filter for air- * 



examination, tttbc. The apparatus is heated and sterilized 

 in a hot-air sterilizer, 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 in from ten to twenty minutes. The sand from 



