THE BACTERIA OF SOIL, AIR, WATER, ETC. 



125 



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

 various devices that have been proposed for the accurate study 

 of the organisms of the air, the Sedgwiclc-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. 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 con- 

 nected 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 mi- 

 croorganisms which are contained in it. Liquefied gelatin or 



Fig. 47. — Sedgwick-Ttjcker Aerobioscope. 



agar 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 culture-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. 



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

 and the ocean always contains bacteria. The number of 

 organisms varies greatly in different places and under diiierent 

 conditions. The number of different species found in water is 

 also very large. Ground-water * contains few or no bacteria 



* Ground-water is the water which — originally derived from rain or snow — 

 sinks through superficial porous strata, like gravel, and collects on some under- 

 lying, impervious bed of clay or rock. 



