CHEMICAL AND PHYSICAL NOTES. 1538 
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 thick- 
ness 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 1 kilogramme per square centi- 
metre, and the tension of the air in contact with the water is also 
1 kilogramme per square centimetre. Let the layer of water be 
heated. When it arrives at a temperature of 99°:1 C. the tension 
of its vapour is exactly 1 kilogramme per square centimetre, and 
it is theretore equal to the atmospheric pressure, and any further 
supply of heat will cause the water to boil at the temperature 
of 99°-1C. Let the water be cooled down again ; and let the density 
of the earth be reduced by one-half. Then the height of the baro- 
meter will remain unaltered at 735°5 mm., but the pressure of the air 
will be halved and will be only 0°5 kilogramme per square centimetre. 
Let the water now be warmed. When the temperature of the water 
arrives at 80°°9 C. the tension of its vapour is exactly 0°5 kilo- 
gramme per square centimetre, and any further supply of heat will 
cause the water to boil at this temperature. 
It is evident then that if we know the relation between the ten- 
sion of aqueous vapour in kilogrammes per square centimetre and 
the temperature, we have in the boiling-point of water a means of 
measuring the true pressure of the atmosphere in which it is 
boiling. 
But we have seen that the pressure of the atmosphere is propor- 
tional to the quantity of the air that is in the column which rests on 
the surface of the boiling water, and to the force of gravity or the 
earth’s attraction which pulls it towards its centre. Suppose the 
force of gravity constant, the boiling-point of water will be the 
