BAGTEBIOLOGIOAL AIR ANALYSIS. 505 



rolled tubes. It consists in dividing the tube by lines 

 into four or six longitudinal areas, which are subdivided 

 by transverse lines drawn about 1 or 2 cm. apart. The 

 lines may be drawn with pen and ink. They need not 

 be exactly the same distance apart nor exactly straight. 

 Beginning with one of these squares at one end of the 

 tube, which may be marked with a cross, the tube is 

 twisted with the fingers, always in one direction, and 

 the exact number of colonies in each square as it 

 appears in rotation is counted, care being taken not to 

 count a square more than once; the sums are then added 

 together, and the result gives the number of colonies 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 diffi- 

 cult to eliminate: it is assumed that each colony repre- 

 sents the outgrowth from a single organism. This is 

 probably 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 shaking 

 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. 



Bacteeiologicai; Air Analysis. — Quite a number 

 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, 



