88 ELEMENTS OF SCIENCE 



vapour as in boiling or " ebullition." Pressure produces 

 a definite effect on this process. Thus the weight of 

 the atmosphere causes water at the sea-level to need 

 a greater supply of heat to boil than is required on 

 a very elevated mountain, where its pressure is 

 necessarily much less. By lowering temperature, the 

 vapour will again assume the liquid condition as 

 before said. 



But heat does not only act as a transformer of bodies 

 from one state to another, it also exercises one very 

 notable effect on bodies which remain in an unchanged 

 condition, whether that be solid, liquid or aeriform. It 

 makes them expand, as we see in the thermometer, where- 

 in heat causes the liquid it contains to rise in a vertical 

 tube, owing to the expansion it produces in that liquid. 

 Heat applied to a gas enclosed in a vessel, cannot, of 

 course, expand it (beyond what may be allowed by the 

 expansion of the vessel itself from heat) on account of its 

 boundary, but by its very gaseous nature, it is always 

 expanded, and presses upon every part of the vessel 

 enclosing it, however low the temperature supposing 

 it is not low enough to turn the gas into a liquid. But 

 it will tend to make the gas press more energetically 

 against the vessel containing it, and may cause that 

 vessel to burst. 



The whole world is continually and everywhere under 

 the influence of heat, and when heat is passing from one 

 body to another, the former is said to be of a higher 

 " temperature " than the latter, the temperature of 

 which is raised by the heat it receives from the former. 

 Thus the terms hot and cold imply a relation existing 

 between two bodies as regards their temperature. 

 There is absolutely no such thing as absolute coldness ; 

 cold is but a relative term. 



