62 SMITHS' INTERMEDIATE CHEMISTRY 



of mercury is depressed 4.6 mm., at 10 the vapor pressure becomes 

 9.2 mm., at 20 17.4 mm. (see Appendix IV). At 100 the level of 

 the mercury in the tube would be depressed 760 mm., and would 

 sink to the level of that in the trough. If a little air is first placed 

 above dry mercury, causing it to fall, the additional depression 

 produced by adding water is the same as if the air had been absent 

 (p. 47). 



Steam at 100, subjected to a pressure over one atmosphere, is 

 condensed to water. Steam cannot be condensed by pressure, 

 however, if the temperature is over 374, its critical temperature. 

 At 100 the steam occupies nearly 1600 times the volume of an 

 equal weight of water. 



Steam is a perfectly invisible gas. The visible cloud of fog, 

 issuing from a valve when steam escapes, is composed of mi- 

 nute drops of water formed by condensation. 



Molecular Relations of Liquid and Vapor. When the 

 water was introduced above the barometric column, the vapor, or 

 gaseous water, could have resulted only from the spontaneous 

 motion of the molecules in the liquid. Some of the molecules, 

 moving near the surface, went off into the space above the water 

 and became gaseous. To be consistent, we must also conclude 

 that the vapor above the water is not composed of the same set of 

 molecules one minute as it was during the preceding minute. 

 Their motions must cause many of them to plunge into the liquid, 

 while others emerge and take their places. When the water is 

 first introduced, there are no molecules of vapor in the space at all, 

 so that emission from the water predominates. The pressure of 

 the vapor increases as the concentration of the molecules of vapor 

 becomes greater, hence the mercury column falls steadily. At the 

 same time the number of gaseous molecules plunging into the 

 water per second must increase in proportion to the degree to 

 which they are crowded in the vapor. The rate at which mole- 

 cules return to the water thus begins at zero, and increases 



