ELASTICITY OF CARBONIC ACID. 75 



half of the dissolved gas escapes from the liquid, 

 because this gas, if it were not dissolved at 

 all, would occupy twice as much space as the water 

 fills, while the water can still only dissolve and 

 keep in solution a bulk of gas equal to its own. 



Let us now suppose, that water, which in the 

 depths below has become more or less strongly 

 charged with carbonic acid, reaches, in the channel 

 that gives it vent to the surface, a height, at which 

 the pressure from above is less than the elasticity (or 

 effort to return to the gaseous state) of the car- 

 bonic acid. The gas now releases itself, and, 

 pushing before it a certain quantity of the liquid, 

 forces its way, mixed with the water, up the vent- 

 shaft j — just as you see it doing when a bottle of 

 champagne is opened. There are here again, you 

 observe, all the conditions, whose presence in the 

 hot springs of Iceland I have explained; with 

 this difference only, that there steam, and here 

 another gas, carbonic acid, is the moving power. 

 You will now understand that the springs may 

 be forced up by the joint power of water-pressure, 

 and of the expansion of the gas, or even by the 

 latter agency alone. And this will probably be de- 

 termined by local circumstances, such as the depth 

 from which the water rises, the width and shape of 

 the channel, the quantity of gas held in solution, 

 the temperature, &c. The water so sent up, may 

 rise above the surface of the earth in a uniform 



