PROPERTIES OF GASES 69 



another important relation, which is, however, another way of expressing 

 those already noticed. It is found that, when there is no alteration of 

 temperature, the product obtained by multiplying the volume of a 

 given quantity of gas by the pressure to which it is subjected is always 

 the same, or remains constant. 



These facts were discovered by Boyle, and are included in what is 

 known as Boyle's Law. It can be expressed by saying that when 

 the temperature remains the same, the volume of a given mass of gas 

 varies inversely as its pressure. Or, what is the same thing, the tem- 

 perature remaining the same, the product of the pressure into the volume 

 of a given mass of gas is constant. 



But it has been seen that if the volume occupied by a given weight 

 of a substance is increased, its density is decreased, and if the volume 

 is decreased, its density is increased. Therefore, by decreasing the 

 volume of the enclosed air in the above experiment, its density is 

 increased. The increase of density and the increase of pressure are 

 proportional to one another. It is not difficult to apply these facts 

 to the case of the atmosphere. It has been learnt that the pressure 

 of the atmosphere decreases as we ascend, and we are now able to add 

 that its density decreases also and at the same rate. Therefore the 

 densest atmosphere is that at the surface of the earth, leaving out, 

 of course, the air of mines and other cavities below the surface, where 

 the air is denser still. The air gets less dense, or rarer, as we leave the 

 earth's surface, until eventually it becomes so rare that its existence 

 is practically not appreciable. 



QUESTIONS ON CHAPTER XII. 



49. How could you prove that air has weight ? How would you find 

 the weight of the air contained in a flask ? 



50. Distinguish between " mass " and " weight." Explain how you 

 would find, by experiment, the weight of a cubic metre of the air in a room 



51. Describe briefly two methods for obtaining the weight of a litre of 

 air, and show which is the more likely to give an accurate result. 



52. Explain how you would find the weight of air contained in a room. 

 How would you expect this to vary with the height of the barometer and 

 thermometer ? 



53. Give an account of the construction of a barometer, and explain 

 how it measures the pressure of the atmosphere. 



Calculate the pressure of the atmosphere when the mercury in a baro- 

 meter stands at 76-2 cm. 



54. Describe how you would proceed to construct and set up a simple 

 barometer, indicating the chief precautions to be observed. 



