Chap, xxxvn.] EBULLITION. 473 



upwards in the liquid, probably because, coming in 

 contact with colder layers of water, they become con- 

 densed. It is thought that it is the collapsing of 

 such bubbles that causes the singing sound heard 

 before the liquid is at the boiling point. When the 

 boiling point is reached, the vapour escapes from the 

 liquid and gives rise to the commotion called ebulli- 

 tion. In order that the vapour may escape into the 

 atmosphere, its tension must be equal to that of the 

 atmosphere. Different liquids have different boiling 

 points. Distilled water is 100 C., ether 37, alcohol 

 79, mercury 353, sulphur, 440. 



Although the application of heat be continued 

 after the boiling point has been reached, the tempera- 

 ture of the liquid does not rise. The additional heat 

 is consumed in the conversion of the liquid into the 

 gaseous state. Thus heat disappears in the passage 

 from the liquid to the gaseous state, as well as in the 

 passage from the solid to the liquid state. To express 

 this disappearance, the phrase latent heat of vapour 

 is employed. For this reason a kettle will not be- 

 come red hot so long as it contains water, the tem- 

 perature not being able to exceed 100 C. Similarly, 

 water may be boiled in a capsule of paper, because 

 the temperature of 100 C. is not sufficient to ignite 

 the paper. For the conversion of unit mass of water 

 at 100 C. into vapour, 536 units of heat measured on 

 the centigrade scale, are required, or about 965 units 

 Fahrenheit. 



The boiling point is affected by various circum- 

 stances. The presence of saline bodies in solution 

 raises it. Thus, 7*7 per cent, of common salt raises 

 the boiling point 1 C. ; 39*7 per cent, raises it 8. 

 This is supposed to be due to the force of cohesion 

 exerted by the salt molecules on the particles of 

 water. The fact that the boiling point is affected by 

 the nature of the vessel in which the water is placed 



