GRAVITATION. 31 



the liquid used, the length of the tube required must be greater for lighter 

 liquids. As water is 13.6 times lighter than mercury, the height of a water 

 column to balance the atmospheric pressure is 13.6 times 30 inches, or about 34 

 feet, which would therefore be the height of the column of water required. 



Changes in the atmospheric pressure. The height of the mer- 

 cury column in a barometer is not the same at all times, but varies 

 within certain limits. These variations are due to a number of causes 

 disturbing the density of the atmosphere, and are chiefly atmospheric 

 currents, temperature, and the amount of moisture contained in the 

 atmosphere. 



As the height and with it the density of the atmosphere diminishes 

 gradually from the level of the sea upward, the height of the mercury 

 column will be lower in localities situated at an elevation. This 

 diminution of pressure is so constant that the barometer is used for 

 estimating elevations. 



Influence of pressure on state of aggregation. We have seen 

 that the volume of a substance, and, more especially, of a gas, depends 

 upon pressure and temperature, an increase of pressure or decrease of 

 temperature causing the volume to become smaller. We learned also 

 that liquids may be converted into gases, and that this conversion ' 

 takes place at a certain fixed temperature called the boiling-point. 

 This point, however, changes with the pressure. An increased pres- 

 sure will raise, a decreased pressure will lower, the boiling-point. 



Thus water boils at the normal pressure of one atmosphere at 100 0. (212 

 F.), but it will boil at a lower temperature on mountains in consequence of the 

 diminished atmospheric pressure. If the pressure be increased, as, for instance, 

 in steam-boilers, the boiling-point will be raised. Thus the boiling-point of 

 water under a pressure of two atmospheres is at 122 C. (251 F.), of five atmos- 

 pheres at 153 C. (307 F.), of ten atmospheres at 180 C. (356 F.) 



QUESTIONS. 21. What is gravitation? 22. Mention some phenomena 

 caused by gravitation ? 23. Give a definition of weight. 24. What is specific 

 weight ? 25. Name the substances adopted as standards for the determination 

 of specific gravities of solids, liquids, and gases. 36. What is the use made of 

 hydrometers, and on what principle is their construction based? 27. Explain 

 construction and use of the mercury barometer. 28. Mention some of the 

 causes which have an influence upon the height of the mercury column in 

 the barometer. 29. What is the atmospheric pressure upon a surface of five 

 square feet? 30. State the relation between boiling-point, temperature, and 

 pressure. 



