io8 Chemical and Physical Notes 



of the increased tension of the air at its surface, and the water 

 will again boil. 



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

 the atmosphere and the tension of the air in contact with the 

 boiling water will be reduced. The vapour tension of the 

 water will then be higher than that of the air in contact with 

 it, the water will be super-heated and will boil for a time 

 without any further supply of heat, the super-heat in the water 

 being sufficient. When this is exhausted, the temperature of 

 the water will be reduced to that at which its vapour tension 

 is the same as the reduced tension of the air in contact with 

 it, and with a further supply of heat it will continue to boil at 

 this temperature. 



During these changes of the force of gravity and con- 

 sequent changes of the boiling-point of water, the height of the 

 barometer has remained without change, because the changes 

 of force of gravity act equally on both limbs of the siphon. 



In the same way a balance which is loaded with a litre of 

 water on the one pan and a kilogramme of platinum on the 

 other remains in equilibrium, whatever changes may be made 

 in the force of gravity. If, however, the litre of water were 

 weighed on a .s^Vz^-balance, its weight would vary in the same 

 proportion as the force of gravity. Its weight on a spring- 

 balance is the measure of the power which it has to overcome 

 a certain constant resistance, the resilience of the spring. The 

 whole of this power is conferred on it by the force of gravity. 

 By virtue of its mass alone it has no such power. Hence, in 

 the case considered, when the density of the earth is supposed 

 to be halved, the weight of the litre of water would be halved 

 also, and it would be registered as 500 grammes on the scale 

 of the spring-balance. The same would apply to the kilo- 

 gramme of platinum. The spring-balance would give its 

 weight as only 500 grammes, and it is evident that under the 

 altered conditions of gravity they would still balance each 

 other in the opposite pans of a pair of scales. Like the 

 hypsometer, the aneroid barometer measures the pressure of 

 the air, not the height of the barometer. It is a spring-balance, 

 whilst the mercurial barometer is a pair of scales. 



