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in the conditions produces', never a great or sharp change, but 

 always, and instantly, a corresponding small change in the state of 

 the system. The change in the conditions accomplishes this by 

 favoring or disfavoring one of the two opposing tendencies. Thus, 

 for example, when the temperature of a liquid is raised, the motion 

 of its molecules is increased, the rate at which they leave its sur- 

 face becomes greater, the vapor tension increases and, hence, a 

 greater concentration of vapor can be maintained. The system, 

 therefore, quickly reaches a new state of equilibrium in which a 

 higher vapor pressure exists. 



In the preceding illustration, the evaporating tendency was 

 favored by a rise in temperature. As an example of a change in 

 conditions disfavoring one tendency, take the case where the liquid 

 is placed in an open, shallow vessel. Here the condensing ten- 

 dency is markedly discouraged, for there is less chance of return 

 of the emitted molecules. Hence complete evaporation finally 

 takes place. Elevation of the temperature hastens the process. 

 A draft insures the diffusion of the vapor away from the surface 

 of the liquid, and has therefore the same effect. The two methods 

 of assisting the displacement of an equilibrium, and particularly 

 the second, in which the opposed process is weakened and the 

 forward process triumphs solely on this account, should be noted 

 carefully. They are applied with surprising effectiveness in the 

 explanation of chemical phenomena. 



The States of Matter. Most substances are known in at 

 l?ast three different states, namely, a crystalline (or solid), a 

 liquid, and a gaseous form. There is no magic about the number 

 three, however. Very many substances are known in more 

 states than three. Thus sulphur has a vapor state, two liquid 

 states, and several different crystalline forms. There are no 

 fewer than five forms of ice, different in physical but identical in 

 chemical properties. 



When we wish to transform a substance from one state to 



