186 BACTERIOLOGY. 



quantitative chemical analysis as well as a thorough 

 inspection of the supply ; at the same time the character 

 of the organisms should be subjected to most careful 

 study. 



Baoteeiological 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 pro- 

 cedure. An objection raised against it is that organisms 

 may be lost, and not come into the calculation, by pass- 

 ing 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 

 appear, on the whole, to give the best results. Petri 

 recommends the aspiration of a measured volume of air 

 through glass tubes into which sterilized sand is packed. 

 (Fig. 30.) When the aspiration is finished the sand 

 is mixed with liquefied gelatin, plates are made, and 



