BACTERIOLOGICAL EXAMINATION OF THE SOIL. 203 



solid substances, such as dirt, etc., loaded with bacteria. These 

 may travel in suspension in the air for a considerable distance. 



The Sedgwick-Tucker method is perhaps the best procedure 

 for the examination of air. 



For this purpose an apparatus known as the aerobioscope 

 (Fig. 74) is required. In this apparatus a certain amount of 

 sterile dry granulated sugar is introduced into the narrow 

 part of the tube at d ; and at a a small roll of fine brass 

 wire-gauze is inserted in order to provide a stop for the 

 filtering material which is placed over it, as, for example, 

 the sugar; then by means of an air-pump a certain defi- 

 nite quantity of air is sucked through the aerobioscope, 

 after which 'the apparatus is closed with sterile cotton at 



FIG. 74. 



The Sedgwick-Tucker aerobioscope. (Abbott.) 



b and at c, and by gentle tapping, the contaminated sugar 

 is forced into the larger portion of the tube at e; when 

 this is accomplished 20 c.c. of liquefied sterile gelatin are 

 poured in the larger part of the tube, the sugar dissolved in 

 this gelatin, and an ordinary Esmarch tube made. The 

 colonies may be counted as in an Esmarch tube, and pure 

 cultures of the isolated colonies may be made on other plates. 



THE BACTERIOLOGICAL EXAMINATION OF THE SOIL. 



In the study of the soil for microorganisms special instru- 

 ments for collecting the soil at different depths have been in- 

 vented. C. Fraenkel's apparatus is perhaps the most useful. 



Small fragments of the soil to be examined should be dis- 

 solved in liquid gelatin or agar, plates made, and the colonies 

 counted as for the examination of water. 



The objection to this method, however, lies in the fact that 



