292 Wells & Foote — One Hundred Years of Chemistry. 



the latter if always in excess may be regarded as con- 

 stant in concentration, and the law: takes on a simpler 

 aspect in applying" only to the concentrations of the 

 gaseons substances. For example, in the reversible 

 reaction 



3Fe + 4H 2 O^Z±Fe 3 4 + 4H 2 , 



which takes place at rather high temperatures, a definite 

 mixture of steam and hydrogen at a definite temperature 

 will cause the reaction to proceed with equal rapidity in 

 both directions, thus maintaining a state of equilibrium, 

 provided that both iron and the oxide are present in 

 excess. If, however, the relative concentrations of the 

 hydrogen and steam are changed, or even if the tempera- 

 ture is changed, the reaction will proceed faster in one 

 direction than in the other until equilibrium is again 

 attained. 



The principle of mass-action also explains why it is 

 sometimes possible for a reversible reaction to become 

 complete in either direction. For instance, in connec- 

 tion with the reaction that has just been considered, if 

 steam is passed over heated iron and if hydrogen is 

 passed over the heated oxide, the gaseous product in each 

 case is gradually carried away, and the reaction contin- 

 ually proceeds faster in one direction than in the other 

 until it is complete, according to the equations 



3Fe + 4H 2 G > 3Fe 3 4 + 4H 2 and 



Fe 3 4 + 4H 2 > 3Fe + 4H 2 0. 



Many other well-known and important facts, both 

 chemical and physical, depend upon this law. It explains 

 the circumstance that a vapor-pressure is not dependent 

 upon the amount of the liquid that is present; it also 

 explains the constant dissociation pressure of calcium 

 carbonate at a given temperature, irrespective of the 

 amounts of carbonate and oxide present; in connection 

 with the ionic theory, it furnishes the reason for the 

 variable solubility of salts due to the presence of elec- 

 trolytes containing ions in common; and it elucidates 

 Henry's law which states that the solubilities of gases are 

 proportional to their pressures. 



Ostwald, more than any other chemist, has been instru- 

 mental in making general applications of this law, and he 

 made particularly extensive use of it in connection with 



