CHAPTER X. 

 CHANGE OF STATE LIQUID VAPOUR. 



General Account of Evaporation Vapour- Pressure Boiling Delayed Boiling 

 Condensation on Nuclei Measurements of Vapour-Pressure Determination of 

 Vapour Density Density of Saturated Vapour Measurements of Latent Heat 

 of Vapours Specific Heat of Saturated Vapour Spheroidal State. 



Change Of State. We are accustomed to find each substance occur- 

 ring most commonly in one particular state of matter. Iron, for ex- 

 ample, is usually solid, oil liquid, and air gaseous. But we are also 

 familiar with the change of the same kind of matter from one state to 

 another, which is effected by supplying or withdrawing heat, and experi- 

 ment leads us to believe that, unless chemical change intervenes, every 

 substance may be made to assume any one of the three states, even 

 though our present experimental arrangements may be insufficient to 

 accomplish this. The change as we add heat is, in general, from solid to 

 liquid, and from liquid to gas, though it may be from solid to gas without 

 the intervening step. There are also apparent exceptions, as in the case 

 of the preparation of solid red phosphorus by heating ordinary molten 

 phosphorus. But we probably have here an absorption of energy 

 accompanying quite a different arrangement of the molecules a change 

 of state of another kind. We may take it, therefore, as a rule that the 

 three states solid, liquid, gas are in ascending order as regards the 

 quantity of energy possessed, and the quantity required to effect the 

 change from one state to the next is usually large. If, for example, 

 we take a quantity of ice below C. and supply heat to it, the tempera- 

 ture rises steadily to 0. There is then a pause while melting takes 

 place, a very considerable quantity of heat being absorbed merely to 

 effect the change from ice to water while the temperature remains 

 steady. This heat is said to be latent, a term which was given on the 

 supposition that the general effect of heat was to raise the temperature 

 of bodies, whereas this heat is not affecting the temperature. From this 

 point of view the term is a very good one, though, of course, the heat is 

 not latent in any other sense, as its presence is quite evident in change 

 of physical state. 



The ice being melted, the water again rises steadily in temperature 

 till it begins to boil, turning rapidly into steam or water-gas, when there 

 is another pause, and a still larger quantity of " latent " heat is required 

 merely to effect the change from water to steam without rise of tempera- 

 ture. But besides this rapid change at boiling, when the water is in an 

 open vessel a gradual change into steam takes place at the upper surface 

 even at ordinary temperatures, a change which is more rapid as the 

 temperature rises. This change, which is termed evaporation, even takes 



