252 BACTERIOLOGY. 



per cent, solution of carbolic acid upon it and drying 

 with a sterile cloth. 



The technique of making plate cultures, of counting 

 the number of colonies, and of isolating and identi- 

 fying pathogenic species are described under the special 

 chapters devoted to these subjects. A point to be re- 

 membered is that about double the number of colonies 

 usually develop at 20 C. as at blood heat (37 C.), 

 many water bacteria not growing at body-temperature. 

 A convenient flat flask with ruled surface (Fig. 38) 

 has been devised to take the place of the Petri dish 

 when the number of bacteria only and not the varieties 

 are wanted. In these there is no danger of contami- 

 nating air-organisms entering during transportation. 

 The stopper can be graduated to hold one c.c. 



The Bacteriological Examination of Air. Saprophytic 

 bacteria are always present in considerable numbers in 

 the air except far out at sea or on high mountains. 

 They are more abundant where organic matter abounds 

 and in dry and windy weather. Pathogenic bacteria, 

 on the other hand, are only occasionally present in 

 the air. The practical results obtained from the ex- 

 amination of air for pathogenic bacteria have been 

 slight. We know that at times they must be in the 

 air, but unless we purposely increase their numbers 

 they are so few in the comparatively small amount of 

 air which it is practicable to examine that we rarely find 

 them. Examination of dust, however, in hospital 

 wards and sick-rooms, in places where only air infec- 

 tion was possible, have revealed tubercle bacilli and 

 other pathogenic bacteria. 



The simplest method of searching for the varieties 

 of bacteria in the air and their number in any place is 



