458 BACTERIOLOGY. 



in the tube. This method may be facilitated by the 

 use of a hand-lens. 



In all these methods there is one error that is difficult 

 to eliminate : it is assumed that each colony represents 

 the outgrowth from a single organism. This is prob- 

 ably not always the case, as there may exist clumps of 

 bacteria which represent hundreds or even thousands of 

 individuals, but which still give rise to but a single 

 colony this is usually estimated as a single organism 

 in the water under analysis. 



Where grounds exist for suspecting the presence of 

 these clumps, they may in part be broken up by shak- 

 ing the original water with sterilized sand. 



What has been said for the bacteriological examina- 

 tion of water holds good for all fluids which are to be 

 subjected to this form of analysis. 



BACTERIOLOGICAL AIR ANALYSIS. Quite a num- 

 ber of methods for the bacteriological study of the air 

 exist. 



In the main they consist either of allowing air to pass 

 over solid nutrient media (Koch, Hesse) and observing 

 the colonies which develop upon the media, or of filter- 

 ing the bacteria from the air by means of porous and 

 liquid substances, and studying the organisms thus 

 obtained. (Miguel, Petri, Strauss, Wiirz, Sedgwick- 

 Tucker.) 



The former methods have given place almost entirely 

 to the latter for reasons of greater exactness possessed 

 by the latter. 



In some of the methods which provide for the filtra- 

 tion of bacteria from the air by means of liquid sub- 

 stances, a measured volume of air is aspirated through 

 liquefied gelatin ; this is then rolled into an Esmarch 



