232 



Domestic Science 



EXPERIMENT 92. Expose a little clear lime-water 

 on a watch-glass for about twenty minutes. At the 

 end of that time the surface will be covered with a thin 

 bluish-white skin. This consists of precipitated chalk, 

 the particles of which are too fine to break through the 

 water-surface and fall to the bottom of the glass. 



The formation of this skin is evidence of the existence 

 of carbon dioxide in the air which has come in contact 

 with the surface of the lime-water. 



159. A more pronounced result may be obtained 

 by aspirating a large quantity of air through lime- 

 water. 



EXPERIMENT 93. Fit a tall gas -jar, containing a 

 little clear lime-water, L, with a 

 well-fitting cork, C, having two 

 holes bored through it. The tubes 

 / and pass through the holes and 

 fit them closely. 



The tube is connected with 

 an aspirator (see Fig. 72) or a 

 water-pump. By running water 

 from the aspirator (or through the 

 pump) the air is exhausted from 

 G, and the pressure inside the jar 

 thereby lessened. The atmospheric 

 pressure is not altered, and hence 

 air from the room is forced down 



Fig. 74. 



tube / and bubbles up through the lime-water. By 

 passing sufficient air through the liquid, a considerable 

 quantity of the white precipitate of calcium carbonate 

 may be obtained. 



It will be found that the air must be aspirated 

 through the lime-water for some 20 to 30 minutes in 

 order to produce this result, showing that the proportion 



