BACTERIA IN THE AIR. 



621 



Glass tubing having a diameter of about five millimetres is used in 

 making these tubes, and from one to two grammes of powdered sugar 

 is a suitable quantity to use as a filter. The whole apparatus is steril- 

 ized for an hour at 150 C. in a hot-air oven after the pulverized 

 sugar has been introduced. Before using it will be necessary to 

 pack the sugar against the supporting plug a by gently striking the 

 lower end of the tube, held in a vertical position, upon some horizon- 

 tal surface ; and during aspiration 

 the tube must remain in a vertical 

 position, or nearly so, in order that 

 the sugar may properly fill its entire 

 calibre. The aspirator is attached to 

 the lower end of the tube by a piece 

 of rubber tubing. When the tube B 

 is used the sealed extremity is broken 

 off at the moment that the aspirator 

 is set in action, and it is again sealed 

 in a flame after the desired amount 

 of air has been passed through the 

 filter. The next step consists in dis- 

 solving the sugar in distilled water 

 or in liquefied gelatin. To insure 

 the removal of all the sugar the cot- 

 ton plug a may be pushed out with a 

 sterilized glass rod, after removing b 

 with forceps. From fifty to five hun- 

 dred cubic centimetres of distilled 

 water, contained in an Erlenmeyer 

 flask and carefully sterilized, may be 

 used, the amount required depending 

 upon circumstances relating to the 

 conditions of the experiment. By 

 adding five or ten cubic centimetres 

 of this water, containing the sugar 

 and microorganisms arrested by it, 

 to nutrient gelatin or agar liquefied by heat, and then making Es- 

 march roll tubes, the number of germs in the entire quantity is easily 

 estimated by counting the colonies which develop in the roll tubes. 



Sedgwick and Tucker, in a communication made to the Boston 

 Society of Arts, January 12th, 1888, were the first to propose the use 

 of a soluble filter of granulated sugar for collecting atmospheric 

 germs. Their complete apparatus consists of an exhausted receiver, 

 from which a given quantity of air is withdrawn by means of an air 

 pump. A vacuum gauge is attached to the receiver, which is coupled 



FIG. 192. 



FIG. 193. 



