AIR ANALYSIS. 437 



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 filtering 

 the bacteria from the air by means of porous and liquid 

 substances, and studying the organisms thus obtained. 

 (Miguel, Petri, Strauss, Wiirz, Sedgwick.) 



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 

 tube, and the number of colonies counted, just as was 

 done in the water analysis. This is the simplest 

 procedure. An objection raised against it is that 

 organisms may be lost, and not come into the calcula- 

 tion, by passing through the medium in the centre of 

 an air-bubble without being arrested by the fluid — 

 an objection that appears more of speculative than of 

 real value. 



The methods of filtration through porous substances 



