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.e. 
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 
