Minor Constituents of Air 233 



of carbon dioxide contained in ordinary air is extremely 

 small. As an interesting corollary to the above experi- 

 ment, perform 



EXPERIMENT 94. Remove the connection between 

 tube O and the aspirator. Uncork the jar and pour 

 out the lime-water. To clean the inside of the jar from 

 any particles of chalk that may adhere to it, pour in a 

 little dilute hydrochloric acid, shake the jar, and pour 

 away the acid. Rinse the jar with several quantities 

 of water, and then pour in a fresh supply of lime-water. 

 Insert the cork and delivery tubes. Blow air from the 

 lungs for a few seconds through tube /. The prompt 

 production of a white precipitate affords evidence that 

 the proportion of carbon dioxide in expired air is much 

 greater than that existing in ordinary air. 



The observation made in the last experiment explains 

 why the proportion of carbon dioxide in the air of 

 a living-room which is insufficiently ventilated soon 

 becomes considerably higher than that contained in 

 ordinary air. 



160. To show the presence of water-vapour in the 

 air we may either cause some of the vapour to condense 

 and hence become visible as water, or may cause some 

 substance which has a strong attraction for water to 

 remove some of the latter from the air. 



EXPERIMENT 95. Nearly fill a small clean dry beaker 

 with sodium thiosulphate crystals. (This is the sub- 

 stance known to the photographer as " hypo " from its 

 earlier name " sodium hyposulphite".) 



Pour in sufficient water to just cover the crystals. 

 Stir the crystals and water well together with a glass 

 rod for about a minute, and then examine the outside 

 of the beaker. 



The moisture which will be found on the exterior 



