THE DISTRIBUTION OF MICRO-ORGANISMS 177 



In a rough way one may obtain some knowledge of the charac- 

 ter of the organisms in the air of a given locality by removing the 

 cover of a Petri dish containing sterilized gelatin or agar for a 

 few minutes, replacing it, and allowing the organisms to develop. 

 In most cases a large proportion of the growths that appear will 

 be molds. Yeasts are also common, and among the bacteria 

 the micrococci are abundant. Chromogenic varieties are likely 

 to be present. 



A few studies of this character will show that the number 

 of organisms that are present depends chiefly upon whether the 

 air is quiet or has recently been disturbed by drafts, gusts of 

 wind, or sweeping. These facts are of fundamental importance 

 in laboratory work, if we wish to avoid contaminations. Among 

 various devices that have been proposed for the accurate study 



FIG. 82. Sedgwick-Tucker aerobioscope. 



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 sterilized. 

 A definite quantity of air is to be aspirated through the large 

 end, after removing the cotton, and this 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 micro-organisms 

 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 is mixed with 

 the sugar which dissolves. The culture-medium may be spread 



