112 SMITH'S INTERMEDIATE CHEMISTRY 



It is found, as the molecular theory would lead us to expect, 

 that the concentration of the saturated solution of a gas is pro- 

 portional to the pressure at which the gas is supplied (Henry's 

 law). 



This equilibrium, Gas (gaseous) ?= Gas (dissolved), can be 

 reached, naturally, from the other direction, namely by starting 

 with a solution of the gas and a space above the solution contain- 

 ing, at first, none of the gas. The gas leaves the solution until the 

 rates of emission and return become equal. Hence, a gas may be 

 entirely removed from solution by bubbling a foreign gas through 

 the liquid. The bubbles furnish the space to receive the emitted 

 gas, and have a large surface, so that the process goes on rapidly. 

 The bubbles also escape, and carry with them the emitted gas, 

 so that, in this case, there is no re-solution. This is a case of 

 nullifying one of the two opposed tendencies (p. 64). 



When a mixture of two gases is shaken with a liquid, the gases 

 behave independently of each other (Dalton's law, p. 47). Each 

 has the same pressure, and therefore the same solubility, as it 

 would possess if it alone occupied the whole space above the 

 liquid. 



Two Immiscible Solvents: Law of Partition. An inter- 

 esting application of the same ideas may be made to a case which 

 occurs very commonly in chemical work. If we shake up a small 

 particle of iodine with water, we find that it dissolves slowly, 

 giving eventually a saturated but very dilute solution. If now 

 ether in sufficient quantity be shaken with the aqueous solution, 

 the greater part of the iodine will find its way into the ether, and 

 be contained in the brown layer which rises to the top. The proc- 

 ess of removing a substance practically from solution in one sol- 

 vent and securing it in another is called extraction. We find in 

 such cases that neither solvent can entirely deprive the other of 

 the whole of the dissolved substance, if the latter is soluble in 

 both independently. A state of equilibrium is finally reached: 



