THE CHEMISTRY OF THE ATMOSPHERE. 



99 



experiment. All that you require for it is a tumbler of lime- 

 water and a tube. If you breathe through the tube into the 

 lime-water it will soon become milky ; and if you let the 

 tumbler remain for a little time a fine powder will settle on 

 the bottom.* This is calcium carbonate, or chalk, formed 

 by the union of the carbonic anhy- 

 dride that came from your lungs with 

 the calcium hydrate, or lime-water. 

 The experiment can be made more 

 striking by using the simple appara- 

 tus represented in Fig. 30. You can 

 either draw in air through the tube 

 A, and thus let the air that goes into 

 your lungs come through the lime- 

 water, or you can force the air out of 

 your lungs through the lime-water 



by the tube B. If you draw air through the lime-water, it 

 will take a very long time 

 to make it milky, because 

 there is so very little car- 

 bonic anhydride in the air 

 that you breathe in, as you 

 saw in 115 ; but if you 

 throw the air from your 

 lungs into the lime-water 

 by the tube B, it will re- 

 quire only a few breaths 

 to make it decidedly 

 milky. The experiment 

 can be tried in still anoth- 

 er form, as represented in 

 Fig. 31. Here we have lime-water in both vessels. You 



* When a substance thus foils as a sediment in any chemical process, it 

 is said to be precipitated, or is termed a precipitate. 



