THE LUNGS AND RESPIRATION. 219 



The next question that suggests itself is the exact man- 

 ner in which the gases of the air enter the blood, and how 

 those of the blood enter the air. In order to understand 

 this it is desirable to turn aside a little and explain some- 

 what in detail one of the most familiar laws of the physical 

 laboratory, the law which governs the absorption of gases 

 by liquids. 



D ALTON'S LAW OF THE ABSORPTION OF GASES BY LIQUIDS. 



When in any experiment a liquid and a gas are brought 

 together the liquid will at once absorb or dissolve in itself 

 some of this gas, the amount so dissolved depending on the 

 pressure which that gas exerts on the liquid, and varying di- 

 rectly with that pressure. This is not a physiological phe- 

 nomenon but a general physical law, and applies to all liquids 

 and gases. It was named after the physicist who first care- 

 fully proved and formulated the law, and called " Dalton's 

 L,aw of the Absorption of Gases." This law of the absorp- 

 tion of gases announces the observed fact that the amount 

 of gas which any liquid absorbs depends on the pressure 

 which that gas exerts on the liquid and varies directly with 

 it; that is, if the pressure is doubled the amount of gas so 

 dissolved is doubled ; if the pressure is reduced to one-half 

 the amount of gas dissolved is reduced one-half. 



There are many familiar illustrations of this law. For 

 instance, in the manufacture of the familiar soda-water it is 

 deemed desirable to have ordinary water absorb very large 

 quantities of carbon dioxide. Under ordinary pressure 

 water absorbs but little carbon dioxide, but to remedy this, 

 the water to be made into soda-water is subjected in a 

 proper system of tanks to a very great pressure of the car- 

 bon dioxide gas, and in conformity to Dalton's law the water 

 absorbs increased quantities of that gas. The heavier the 

 pressure in such a charged soda-fountain, the more gas will 

 the water absorb. The same thing is illustrated in the bot- 

 tling of mineral waters. Here the ordinary water is sub- 

 jected to a high pressure of carbon dioxide gas, too, and 

 under this high pressure the mineral water dissolves into it- 



