Chemical and Physical Notes 107 



square centimetre. Let the water now be warmed. When 

 the temperature of the water arrives at 8o'9 C. the tension 

 of its vapour is exactly 0*5 kilogramme per square centi- 

 metre, 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 tension 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 

 proportional 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 lower the less air there is in the 

 column above it, or the greater the height of the locality 

 above the level of the sea. If we know the law regulating the 

 relation between pressure of the atmosphere and elevation 

 above sea-level, we have in the boiling-point of water a 

 means of determining the elevation, because the tension of 

 the vapour of the boiling liquid is equal to the tension of the 

 air on its surface, and the tension of the air at the bottom of 

 a column of it is equal to the pressure of the column, and the 

 relation between air pressure and altitude is known. 



If the quantity of air in the column resting on the boiling 

 water remain constant, and the force of gravity, or the 

 attractive force of the earth on a particle at the surface of 

 the water be altered, then the pressure of the column of air 

 on the surface of the water and the tension of the air at the 

 bottom of the column will be altered correspondingly. 



If the force of gravity has been increased, the pressure of 

 the air and the tension of the air at the bottom of the column 

 will be increased ; and if the temperature of the water remain 

 the same the tension of its vapour will be less than the tension 

 of the air in contact with it, and it will cease to boil. If 

 further heat be supplied to it, its temperature will rise to 

 that at which the vapour tension of the water is equal to that 



