io6 Chemical and Physical Notes 



standard units, that is, in millimetres of mercury having a 

 temperature of o C., and subject to the attractive force of 

 gravity which it would experience at the sea-level in latitude 

 45. This force generates in one second a velocity of 98o'6 

 centimetres per second. 



If we consider the layer of air of I mm. in thickness which 

 is contiguous to the surface of the mercury in the outer limb 

 of the barometer, and is pressed down upon it by the weight 

 of the whole atmospheric column above it, we see that the 

 elasticity of this thin layer of air exactly balances the pressure 

 of the air above it and that of the mercury below it. The 

 tension of this layer of air is equal to the atmospheric pressure. 

 Imagine the density of the earth to be reduced by one-half; 

 the height of the barometer is still the same, but the pressure 

 on both sides of the thin layer of air is reduced to one-half; 

 therefore it expands to double its volume, and the thickness 

 of the layer becomes 2 mm., and by consequence its tension 

 has been halved. 



Imagine now a thin layer of water on the surface of the 

 mercury in the outer limb of the barometer. It is pressed 

 down by the weight of the atmospheric column, and it is 

 pressed up by that of the mercurial column. The tension of 

 the air in contact with the water surface is equal to the 

 pressure of the atmospheric column. Let the height of the 

 barometer be 735-5 mm., then taking the mean force of 

 terrestrial gravity at the sea-level in lat 45, the atmospheric 

 pressure is I kilogramme per square centimetre, and the 

 tension of the air in contact with the water is also I kilo- 

 gramme per square centimetre. Let the layer of water be 

 heated. When it arrives at a temperature of 99-! C. the 

 tension of its vapour is exactly I kilogramme per square 

 centimetre, and it is therefore equal to the atmospheric 

 pressure, and any further supply of heat will cause the water 

 to boil at the temperature of 99'! C. Let the water be 

 cooled down again ; and let the density of the earth be 

 reduced by one-half. Then the height of the barometer 

 will remain unaltered at 735-5 mm., but the pressure of the 

 air will be halved and will be only o'5 kilogramme per 



