A Lecture Demonstration Apparatus for Determining the Vapor Pressure of Water

The author has devised this apparatus to show to his classes an experiment in which the vapor pressure of water is determined at temperatures from room temperature up to nearly the boiling point of water. At the right of Fig. 1 is the compact stand holding four barometric tubes. This apparatus is portable and easily carried from the storage cupboard to the lecture table. In addition to being used for this specific experiment, it is also used to demonstrate that it is air that holds up the column of mercury in a barometer. This is easily shown by means of the barometer on the left of the stand. The lower end of this barometer is immersed in mercury enclosed in an eight-ounce bottle from which the air may be removed by means of a water or vacuum pump. The second barometric tube from the left is a standard barometer. The third tube is used to show the vapor pressure of hydrated copper sulfate. It contains a small crystal of hydrated or partially dehydrated copper sulfate on the surface of the mercury. The tube on the right is the one used for this demonstration of the vapor pressure of water. It is enclosed in a water jacket except at the lower end. This tube contains a small drop of water on the surface of the mercury and the space above the mercury therefore contains saturated water vapor