74 



THE LIFE OF THE PLANT 



two following tests serve to distinguish carbonic acid 

 from other gases. If carbonic acid is passed through 

 lime water, i.e. through water in which quicklime has 

 been dissolved, this clear solution becomes clouded with 

 a milky precipitate of chalk, i.e. calcium carbonate, a 

 compound of lime with carbonic acid. I take a flask 

 (fig. 22 A) into which two bent tubes are introduced 

 through the cork ; one of them goes right down into the 

 lime water, while the other, the shorter one, ends above 

 the surface of the liquid. I start by taking the shorter 



Fig. 22. 



tube (a) into my mouth and inhaling air through it. 

 The external air enters through the other long tube, 

 and passes in bubbles through the liquid, which remains 

 transparent. I turn the vessel, take the end of the long 

 tube (b) into my mouth, and exhale the air ; the air 

 repasses in bubbles through the liquid which immediately 

 becomes turbid. In order to prove that the white 

 precipitate at the bottom is really chalk, and that it 

 contains carbonic acid, I add a few drops of vinegar — 

 the precipitate dissolves with effervescence, and these 

 effervescing bubbles of gas are nothing but the carbonic 

 acid which I have just exhaled. 



