MINERAL WATERS, WELL-WATERS, ETC. 219 



filled with gas and water which are evolved, the heat 

 is instantly removed ; a vacuum results from this 

 which causes the return of condensed vapor into the 

 flask. This absorption taking place, it is again heat- 

 ed. A certain quantity of gas is evolved which is 

 added to that which the bell-glass already contains; 

 when this is nearly full, the lamp is removed to 

 determine the new absorption. This operation is re- 

 peated three or four times until the volume of the gas 

 remains stationary. The caoutchouc tube is drawn 

 down the bell-jar into the mercury so that the 

 upper portion contains only the gases which were in 

 solution in the water with a very small quantity of 

 this liquid, which may be greatly diminished by intro- 

 ducing at the end of the operation some fragments of 

 fused chloride of sodium. At length the gases con- 

 tained in the bell-glass are measured, and the pro- 

 portion of carbonic acid is determined by absorbing it 

 by means of potassa. 



Fig. 27. 



UJ 



Sulphuretted hydrogen. The water may contain 

 sulphur in two forms, either combined with hydrogen 



