AIR AND VENTILATION 109 



range from one up to twenty- four hours; at the stated intervals 

 the slides are subjected to microscopical examination. 



A second method is by the use of a Pouchet Aeroscope. This 

 is a funnel-shaped tube or flask the tube of which, drawn to a fine 

 point, ends over a slide moistened with glycerine. The apparatus 

 is enclosed in an air-tight box, and by means of a water aspirator 

 air is sucked through the funnel and deposits its particulate matter 

 on the slide, which can then be examined. Since it is possible to 

 measure the volume of air passing through the funnel, the particu- 

 late matter for any given volume can be estimated. 



Bacterial content of the air can be ascertained by exposing 

 suitable nutrient media on plates and thereafter cultivating by 

 incubation. The quantity of micro-organisms for a given volume 

 of air can be determined by using Hesse's apparatus, whereby 

 a known quantity of air is drawn by aspiration through a cylinder 

 to impinge on nutrient gelatine, which may then be cultivated and 

 the colonies counted. 



The physical state of the atmosphere in a building may be judged 

 by noting if it is moist and oppressive or relatively dry and 

 pleasant. Evidence of excessive humidity is to be noticed on the 

 windows or on any metal work, behind the doors and in corners 

 of the building. Evidence of much moisture in the air of buildings 

 is not in itself a reliable indication of the efficiency of the ventilation 

 when the outside atmosphere is also heavily charged with moisture, 

 that is when its relative humidity is high. If, however, the atmos- 

 phere inside a building is very moist on a cool, dry day, then the 

 ventilation is not efficient. The temperature both inside and out- 

 side the building should be noted, as the degree of difference between 

 them affords a very fair indication of the extent to which the place 

 has been ventilated, provided that artificial heating is not employed. 

 The thermometer, of which there should be at least one in each 

 animal house, must be hung clear of the animals' exhalations and 

 not directly in the path of incoming air. The thermometer should 

 be of the wet and dry bulb combination so that the difference between 

 the two readings may be observed. For more accurate work the 

 readings should be noted twice daily, together with similar readings 

 outside the house, and the relative humidity of each position cal- 

 culated by means of Glaisher's tables (see page 40). 



