BACTERIOLOGIC EXAMINATION OF AIR 175 



black glass plate, contained in a wooden frame. The Petri dish is 

 placed on the black glass. Above it, resting on four blocks, is a 

 transparent glass plate which has been ruled with a diamond into 

 square centimeters and fractions of a square centimeter. The colonies 

 in one cubic square, both on the surface and in the depth of the medium 

 are counted with the aid of a hand magnifying glass or the low-power 

 lens of a compound microscope with a large stage. This is done for 

 a number of squares and the average number of colonies per square 

 centimeter is then calculated. If it has not been done previously, 

 the exact diameter of the lower part of the Petri dish is then measured 

 and the surface of the culture medium contained in the lower dish is 

 calculated according to the formula for the surface of a circle, which 

 is r 2 7i. For example : Counting 5 square centimeters : 



Square No. 1 . | ,,; J Q- 16 colonies 



Square No. 2 ...."..,. , 23 colonies 



Square No. 3 . . . . : . ..'.'. . . . '.'"" . ' ' .' . 14 colonies 



Square No. 4 . . J '.'''; ' . .rvv 1 *. 18 colonies 



Square No. 5 .. .. ..*;,..,>. .,.[> .;,..,, (<*<-!{' ,j[i ,<<,.'' 19 colonies 



Total '".'"'."".''. 90 colonies 



Average per square cm. . t -. t , /f ^ j ?> . . . . -j ' \ ' 18 colonies 



Diameter of lower part of Petri dish .'.-....... . 6 om. 



Hence radius .' '.''". ''.''. >; i ! '. '".'' .' ' / >! V ';' I . 1 '" ! . ; ' < >C{ . 3cm. 

 Hence surface 9X3.14 ,;,- >,]. .-:t !rt ' f ,r? v v!! .;,[. 28.26 cm. 

 This figure multiplied by 18 equals 308.68 colonies 



This means that the Petri dish has developed 309 colonies; and if 1 c.c. of water was 

 mixed with the culture medium, the result shows that this water did contain 309 live bacteria 

 per cubic centimeter. 



Esmarch's Apparatus. Sometimes, in the examination of air, 

 water, etc., plates cannot be poured. In such cases Esmarch role- 

 tubes or other glass tubes coated on the inside with the culture medium 

 must be prepared for the development of the colonies. With tubes 

 the Wolffhiigel counting apparatus is replaced by a special magni- 

 fying glass with tube holder, devised by Esmarch. The outside of 

 the glass tube is divided into a number of equal fields. The colonies 

 in a number of fields are counted, the average is obtained and multi- 

 plied by the total number of fields. In other words, the fields are 

 treated the same as the squares in the preceding example. 



These methods furnish an approximate result only, but one that is 

 accurate enough for the purpose. 



Bacteriologic Examination of Air. When a very exact quantitative 

 bacteriologic examination of air is unnecessary the method practised 

 by Robert Koch will be found both simple and satisfactory. Culture 

 media are poured into Petri dishes and allowed to solidify. The lid 

 of the dish is then removed and the culture medium exposed to the 

 air for a definite period of time (for instance, ten or fifteen minutes). 

 The lid is then replaced and the Petri dish is kept at room temperature 

 or incubated in the usual manner. By this simple method compara- 



