30 INTRODUCTION. 



As the density of the atmosphere diminishes gradually from 

 the level of the sea upwards, the height of the mercury column 

 will be lower in localities situated at an elevation. This dimi- 

 nution of pressure is so regular 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 

 and decrease of temperature causing the volume to become 

 smaller. We also learned 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 pressure will raise, 

 a decreased pressure will lower, the boiling-point. 



Thus water boils at the normal pressure of one atmosphere at 

 100, 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 2 atmospheres is at 122, of 5 atmospheres 

 at 153, of 10 atmospheres at 180. 



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. 



26. 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. 



