YENTILATION 89 



The presence of ammonia in stable air is generally recog- 

 nisable by the senses without any chemical test. Ordinary 

 blotting-paper soaked in a solution of logwood turns purple 

 in the presence of ammonia; papers soaked in Nessler's 

 solution are an exceedingly delicate test. 



Sulphuretted hydrogen may be determined by the use of 

 papers soaked in a solution of lead acetate. 



The amount of watery vapour is ascertained by means of 

 the wet and dry bulb thermometers, or by the hygro- 

 meter. 



The temperature outside and inside the stable should be 

 recorded. If no artificial heating be practised, the greater 

 the difference in temperature the greater the air impurity. 



The microscopical examination of the air is more easily 

 accomplished than the chemical, but gives far less practical 

 information. A simple method is to expose for some hours 

 in the stable a slide moistened with glycerine, to which the 

 floating particles in the air adhere. 



There are other more elaborate methods, such as draw- 

 ing by means of an aspirator a known quantity of air 

 through a tube terminating in a drop or two of glycerine ; or 

 drawing a known quantity of air through a glass tube con- 

 taining nutrient gelatine. In the latter the solid particles 

 adhere, the colonies grow and may readily be counted, and 

 thus the number of living organisms in a known volume of 

 air can be estimated. 



A simple plan, to obtain the invisible vapour of the atmo- 

 sphere in the form of water, and with it the solid particles, 

 is to place some ice in a vessel ; the moisture condenses on 

 its surface and runs down, and may be collected and 

 examined. 



Finally, the examination of the air should be made at 

 night some hours after the stable has been occupied. 



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