LETTEB VI. 



JETS OF GAS AND MUD VOLCANOES. 



The agency of carbonic acid gas in giving force 

 to the flow of springs, is very similar to that of 

 steam. Water, when it is brought in contact with 

 carbonic acid, is able to dissolve a quantity of that 

 gas in bulk somewhat greater than itself. Por 

 instance, one cubic foot of water takes up one 

 cubic foot of carbonic acid, and that without 

 much increasing its own bulk. This proportion 

 of solubility is always the same for carbonic acid, 

 whatever may be its density. Since now this gas, 

 like any other, is condensed in proportion to the 

 pressure it sustains, it follows that water can take 

 up so much the more of it by weight, the greater 

 the pressure from without, under which the so- 

 lution goes on. Eor instance at the depth of six 

 hundred and eighty feet, or under the pressure of a 

 water-column of twenty atmospheres, water can 

 dissolve a portion of carbonic acid twenty times 

 as dense as at the common pressure — a quantity, 

 whose bulk, it is true, is still equal only to its 

 own — but whose weight is twenty times as great 

 as that which it could dissolve in the open air. 

 If now the pressure is lessened to one-half, one- 



