138 MANUAL OF BACTERIOLOGY. 



organisms that are present depends chiefly upon whether the 

 air is quiet or has recently been disturbed by draughts, gusts 

 of wind or sweeping. These facts are of fundamental im- 

 portance in laboratory work, where plate-cultures are being 

 studied, if we wish to avoid contamination of the plates. 

 Among various devices that have been proposed for the ac- 

 curate study of the organisms of the air, the Sedgwick-Tucker 

 aerobioscope is the simplest and most accurate. It consists 

 of a glass cylinder (Fig. 49) a few inches long and an inch or two 

 in caliber with a narrow neck at one end and a narrow tube 

 annealed to the other. A layer of granulated sugar of an 

 inch or more is packed loosely in the narrow tube, and the neck 

 and the end of the narrow tube are plugged with cotton. The 



Fig. 49. — Sedgwick-Tucker aerobioscope. 



instrument is sterilized in the hot-air sterilizer. After removing 

 the cotton a definite quantity of air is to be aspirated through 

 the large end, 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 con- 

 tained in it. Liquefied gelatin or agar in sufficient quantity 

 is introduced into the large end of the instrument by means of 

 a bent funnel; and, after replacing the cotton, it is mixed with 

 the sugar which dissolves. The cuUure-medium is spread 

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

 manner of an Esmarch roll-tube. The bacteria which are 

 filtered out by the sugar develop as so many colonies upon the 

 solidified medium. 



