204 Domestic Science 



The marked difference in the freedom of movement 

 of gaseous as compared with liquid molecules entails 

 again the expenditure in general of an even larger 

 quantity of heat than that demanded by the change of 

 state from solid to liquid. The considerable change of 

 volume occurring during vaporisation necessitates a 

 further expenditure of energy in overcoming the external 

 resistance of the surrounding atmosphere. The latent 

 heat of vaporisation of a liquid is thus accounted for 

 in overcoming the cohesion between the molecules of 

 the liquid and in expanding the volume of the substance 

 against external pressure. 



137. Heat energy, then, resides in bodies as a 

 mode of motion, although this motion is of too delicate 

 a character to be detected as such by the most sensitive 

 instruments at our disposal. Some colour is lent to 

 this view, if we consider the question of what must 

 occur during radiation. In this process motion of 

 some kind is set up by a hot body, whereby heat is 

 transmitted to another through the ether. Little con- 

 sideration is needed to see that it would be against all 

 experience for a body in a state of rest to set up a 

 disturbance whereby this may be effected. Hence our 

 idea that heat is a form of kinetic energy receives some 

 support. 



138. It may be well to expend a little further 

 consideration on the subject of the action of heat upon 

 gases. In Experiment 51 it was demonstrated that a 

 gas, when heat was supplied to it, was able to overcome 

 the pressure of the atmosphere to some extent. What 

 would happen if, instead of expanding against a constant 

 pressure, as is the case in the experiment quoted, the 

 gas was prevented from expanding ? 



EXPERIMENT 74. Pour mercury into the long limb 



