EXAMINATION OF AIR, SOIL, AND WATER. 



143 



be examined by spreading the sand out in a dish and covering it 

 with nutrient gelatine ; or it may be shaken up with sterilised water 

 and pi ate -cultivations prepared. The sand-filter nearest to the 

 aspirator should remain free from bacteria. 



Sedywick and Tucker employ a glass cylinder which is drawn out 

 at one end into a narrow tube to contain sterilised powdered cane 

 sugar. Both ends of the apparatus are plugged with sterilised 

 cotton-wool. By means of an exhausting apparatus a known volume 

 of air is drawn through the tube. The cotton-wool plug is re- 



FIG. 72. SEDGWICK AND TUCKER'S TUBE. 



moved, and liquid gelatine is introduced into the cylinder, the 

 plug is replaced, and the sugar is shaken into and quickly dissolves 

 in the jelly. The cylinder is then treated in the same way as a 

 roll-culture, and set aside for the colonies to develop (Fig. 72). 



Various forms of " aeroscopes " and 



" aeroniscopes " have from 



FIG. 73. POUCHET'S AEROSCOPE. 



time to time been employed. Pouchet's aeroscope consists of a small 

 funnel, drawn out to a point below which is a glass slip coated with 



