136 MANUAL OF BACTERIOLOGY. 



whether the air is quiet or has recently been disturbed by 

 draughts, gusts of wind, or sweeping. These facts are of 

 fundamental importance in laboratory work, where plate- 

 cultures are being studied, if we wish to avoid contamina- 

 tion of the plates. Among various devices that have been 

 proposed for the accurate study of the organisms of the 

 air, the Sedgwick-Tucker aerobioscope is the simplest and 

 most accurate. It consists of a glass tube, one end of 

 which is drawn out so as to be smaller than the other. 

 The small end contains a quantity of fine granulated sugar ; 

 both ends are plugged with cotton, and the instrument is 



FIG. 47- 



Sedgwick-Tucker aerobioscope. 



sterilized. A definite quantity of air is to be aspirated 

 through the large end, after removing the cotton, which 

 may be done by means of a suction-pump applied to the 

 other end, or by siphoning water out of a bottle the upper 

 part of which is connected with the end of the aerobioscope 

 by means of a rubber tube. The sugar acts as a filter and 

 sifts out of the air the microorganisms which are contained 

 in it. Liquefied gelatin or agar may be introduced into 

 the large end of the instrument by means of a bent funnel ; 

 and, after replacing the cotton, it may mix with the sugar 

 which dissolves. The culture-medium may be spread 

 around the inside of the larger portion of the tube after the 

 manner of an Esmarch roll-tube. The bacteria which 

 were filtered out by the sugar will develop as so many 

 colonies upon the solidified medium. 



Bacteria of Water and of Ice. The water of rivers, 

 lakes and the ocean always contains bacteria. The num- 



