126 BACTERIA IN AIR, SOIL, AND WATER. 



plugged with cotton, and the filter is then sterilised for two or three hours at 

 120° C. — a higher temperature is liable to char the sugar. When used, the 

 filter is to be affixed by its lower end to the aspirating pump and kept in an 

 upright position, the upper plug of cotton is now removed and aspiration 

 carried out. When this is ended, the cotton plug, having meantime been 

 placed in a sterile receptacle, is re-inserted, the apparatus disconnected, the 

 sugar by gentle rapping transferred to the upper portion, 15 c.c. of sterile 

 liquefied gelatin poured into the filter, the filter plugged, and the sugar 

 dissolved. The filter is now to be treated as an Esmarch roll-tube, by 

 being rolled on ice until the gelatin sets, when the apparatus is set aside 

 at room temperature for incubation. This method gives very accurate re- 

 sults. 



When it is necessary to examine air for particular organisms, special 

 methods must often be adopted. Thus in the case of the suspected presence 

 of tubercle bacilli a given quantity of air is drawn through a small quantity of 

 water and then injected into a guinea-pig. 



It must be admitted that comparatively little information 

 bearing on the harmlessness or harmfulness of the air is obtain- 

 able by the mere enumeration of the living organisms present, 

 for under certain conditions the number may be increased by the 

 presence of many individuals of a purely non-pathogenic charac- 

 ter. The organisms found in the air belong to two groups — firstly, 

 a great variety of bacteria ; secondly, yeasts and the spores of 

 moulds and of the lower fungi. With regard to the spores, the 

 organisms from which they are derived often consist of felted 

 masses of threads, from which are thrust into the air special 

 filaments, and in connection with these the spores are formed. 

 By currents of air these latter can easily be detached, and may float 

 about in a free condition. With the bacteria, on the other hand, 

 the case is different. Usually these are growing together in 

 little masses on organic materials, or in fluids, and it is only 

 by the detachment of minute particles of the substratum that 

 the organisms become free. The entrance of bacteria into the 

 air, therefore, is associated with conditions which favour the 

 presence of dust, minute droplets of fluid, etc. The presence 

 of dust in particular would specially favour a large number of 

 bacteria being observed, and this is the case with the air in many 

 industrial conditions, where the bacteria, though numerous, may 

 be quite innocuous. Great numbers of bacteria thus may not 

 indicate any condition likely to injure health, and this may be 

 true also even when the bacteria come from the crowding together 

 of a number of healthy human beings. On the other hand, 



