78 HUMAN PHYSIOLOGY 



the lungs that whilst inspiration is really the act of 

 feeding the tissues with oxygen, expiration is an ac- 

 tion devoted to ridding the body of part of the inevit- 

 able waste which attends the continual work of life. 

 If we breathe through a tube into a bottle of lime 

 water we find clear evidence that carbonic acid gas 

 is given forth after each act of expiration. For the 

 water becomes milkier the longer we breathe into it, 

 and ultimately a white precipitate falls down in the 

 bottom of the vessel, this substance being chalk. It 

 has been formed by the addition of carbonic acid to 

 the lime of the water, chalk being chemically known 

 as carbonate of lime. To demonstrate that the air 

 given from the lungs is warmer than the air we 

 breathe in, is an easier matter still. We have only 

 to breathe on a cold pane of glass to notice the ob- 

 scuring of the service, whilst such a simple experi- 

 ment also proves to us that water is exhaled in the 

 breath in the form of vapour which condenses on the 

 glass, a phenomenon familiarly seen in every crowded 

 railway carriage, the windows of which are closed. 

 The function of breathing has therefore to be defined 

 as a double one in which the nutrition of the body is 

 served in the first instance, and the waste of the body 

 partly excreted in the second. 



ABOUT Am. The air we breathe is a mixture and 

 not a chemical compound of two gases, oxygen and 

 nitrogen. Roughly speaking there are about twenty- 

 one parts of oxygen to seventy-nine parts of nitrogen 

 in a hundred parts of air. Absolutely pure air has 

 this composition, but ordinary air, which may for 

 practical purposes be regarded as pure, contains in 

 addition other elements. A certain amount of car- 

 bonic acid is comprised in all air save the very purest, 

 but where the quantity of this gns does not amount 



