82 HUMAN PHYSIOLOGY 



oxygen, and five per cent more carbonic acid than 

 the air inspired. The amount of oxygen consumed 

 in twenty-four hours by an adult has been estimated 

 at about eighteen cubic feet, this applying to a man 

 at rest. The amount of carbonic acid gas given out 

 might be figured forth from the carbon contained in 

 a piece of charcoal weighing about nine ounces, 

 whilst the amount of water given forth from the 

 lungs in a day on the average may be set down at 

 about half a pint. Concerning the amount of air 

 inhaled at each breath thirty cubic inches may be 

 regarded as the average quantity taken in at each 

 inspiration. A like amount will be given forth with 

 the outcoming breath, but by taking a deep breath 

 an additional hundred cubic inches may be drawn 

 into the lungs, this additional amount being expelled 

 along with the thirty cubic inches represented as 

 taken in by an ordinary inspiration. The amount in- 

 haled in quiet breathing (thirty cubic inches) is called 

 in scientific language, tidal air, from the suggestion 

 that it represents the quiet inflow and outflow of the 

 tide. The additional hundred cubic inches taken 

 in a deep breath constitute complemental air, the cor- 

 responding amount expelled being called supplemental 

 air. It must be noted, however, that over and above 

 the deepest breath we can give forth, a certain amount 

 of air remains in the lungs. This amounts to about 

 one hundred cubic inches. It is termed residual or 

 safety air, for the reason that it is required to 

 prevent the chest and lungs from collapsing through 

 the pressure of the external air. 



BREATHING BAD AIR. A great lesson in public 

 and personal health may be taught us by the know- 

 ledge obtained regarding the necessity for securing 

 a supply of pure air in order that our health and 



