EXAMINATION OF AIB. 453 



is true of the various methods employed in the determina- 

 tion of the number of atmospheric germs. If, for instance, 

 all the organisms present in a given volume of air are trans- 

 ferred to water it does not follow that they will all develop 

 upon the gelatin plate which is made from such water. 

 Many of these may be anaerobic germs, others may require 

 the temperature of the body, and again a large number may 

 fail to grow because the medium is not a suitable one. Con- 

 sequently, the results obtained in the examination of air 

 are to be considered as merely relative in value. 



The earliest method of examining- air, that of Pasteur, consisted 

 in breaking' open, in the desired locality, a sterile flask from which 

 the air had been previously expelled by boiling. The air rushing into 

 the vacuum flask carried with it the suspended organisms which then 

 developed in the nutrient mediijm present. This method demonstrated 

 that atmospheric germs were not as numerous as had been supposed, 

 and, furthermore, that they were not evenly distributed throughout 

 the air. Koch endeavored to determine the number and kind of bac- 

 teria present in a given volume of air by- making use of the gelatin 

 plate method. Sterile gelatin, for example, was placed in an open 

 Esmarch dish within a plugged glass cylinder of known volume. The 

 apparatus was sterilized and then taken to the desired locality where 

 the cotton plug was removed. The cylinder was now allowed to re- 

 main open for some minutes to allow the germ-laden air to enter. The 

 plug was then replaced and the apparatus was set aside. The organ- 

 isms present in this confined volume of air soon settled upon the sur- 

 face of the gelatin where they developed forming colonies. The 

 method, though simple, is extremely imperfect and fully as good re- 

 sults can be obtained by exposing an ordinary gelatin plate direct to 

 the air for 10 to 30 minutes. 



Very fair quantitative results can be obtained by means of 

 Hesse's apparatus. This consists of a glass-tube 70 cm. long and 3 or 

 4 cm. in diameter. One end of the tube is closed with a stopper 

 through which passes a short glass tube. This is connected by means 

 of rubber tubing with an aspirating bottle of known volume. The 

 other end of the tube is closed with two rubber caps, the inner one of 

 which has a small central opening. The tube is first sterilized and 

 then about 50 c.c. of gelatin are introduced. The gelatin is then solid- 

 ified in a thin film over the inner wall of the tube thus making a large 

 Esmarch roll-tube. The apparatus is placed in the room or locality 



