Page:International Library of Technology, Volume 93.djvu/58

 of vaporization of any other liquid is the amount of heat required to change 1 pound of it from the liquid to the vapor, state without increase of temperature.

54. When a given volume of water is changed to steam at atmospheric pressure, the volume of steam formed is 1,646 times as great as the original volume of water. Thus, let the cylinder a, Fig. 8, be fitted with a piston b, and assume that the piston is weightless and frictionless. If the space beneath the piston is filled with water having a depth h and heat is then applied until all the water is changed to steam, it will be found that the piston has risen through a height of $$d = 1,645 h$$ or, in other words, the steam occupies 1,646 times the volume of the water from which it is formed. Since the piston is without weight, it follows that the pressure of the steam is constantly the same as the pressure of the atmosphere above the piston, and this is also the pressure on the water, since the pressure on a liquid is always the same as that of the vapor being evolved from it. In the production of this large amount of steam, work is done in pushing back the atmosphere through the distance $$1,645 h$$, and the energy required to accomplish this is included in the latent heat of the steam.

55. Although the boiling point of water is commonly spoken of as 212° F., the actual boiling point is intimately related to the pressure at which evaporation takes place. Under atmospheric pressure at the sea level (14.7 pounds per square inch), the boiling point is 212° F.; but, if the pressure is increased, the point at which the water boils will be raised. At an absolute pressure of 30 pounds per square inch, the boiling point is approximately 250° F.; while at 100 pounds, absolute, the boiling point is 328° F. If the pressure is reduced, the boiling point will be lowered. 56. If a common laboratory flask a, like that shown in Fig. 9, is partly filled with water and boiled until all the air