THE UJNGS AND RESPIRATION. 221 



liquid. This loss of gas in all these cases would continue 

 until finally the pressure of the gas in the liquid itself would 

 sink as low as the pressure of that gas in the atmosphere 

 above the liquid, when things would come to a standstill. 



The phenomenon of boiling is a still further illustration 

 of this same fact. As the water becomes gradually more 

 and more heated the pressure of steam in the liquid rises 

 correspondingly, until finally a point is reached at which the 

 pressure of the steam in the liquid is a little greater than 

 the pressure of the atmosphere above the liquid. As a re- 

 sult of this the water is thrown upward to allow the steam to 

 escape. This throwing up of the water is, of course, the 

 familiar boiling of the liquids. For this reason we ought to 

 expect that water would boil more quickly the lower the at- 

 mospheric pressure. Such is, of course, the case, and water 

 will boil on high mountainous altitudes at very much re- 

 duced temperatures. Finally, it may not be altogether out 

 of place to call renewed attention to the fact that such a gas 

 absorbed in or dissolved by a liquid is not held in that liquid 

 in the form of bubbles, but is thoroughly dissolved in it and 

 to the eye invisible. Thus, a bottle of heavily charged 

 mineral water will appear without a bubble in it as long as 

 it remains corked, and yet at the moment of opening, in- 

 numerable bubbles will at once form throughout the liquid, 

 sometimes with such rapidity as to cause the whole liquid 

 to froth. 



With this little side excursion into the realm of physics, 

 let us turn again to the condition of things in the lungs 

 and see the application there of Dalton's law. 



A difficulty at once confronts us. The air is not a 

 single gas. It is made up of at least three gases. It con- 

 tains about four-fifths nitrogen, one-fifth oxygen, and then 

 traces of CO 2 . The experiments of the physicist again 

 help us out here, for in all cases where a liquid is subjected 

 to a mixture of gases each gas acts independently of all the 

 others; that is, the amount of one gas which will be ab- 

 sorbed by a liquid is not in any way influenced by the 



