EFFECTS OF HEAT. 



If a body in the state of vapour lose heat in sufficient quantity, 

 it will pass into the liquid state. Thus, if a certain quantity of 

 heat be abstracted from steam, it will become water. 



This change is called condensation ; because, in passing from 

 the vaporous to the liquid state, the body always undergoes a 

 very considerable diminution of volume, and therefore becomes 

 condensed. 



7. Heat, when imparted to bodies in a certain quantity, will in 

 some cases render them luminous. 



Thus, if metal be heated to a certain degree, it will become red' 

 hot; a term signifying merely that it emits red light. This 

 luminous state, which is consequent on the accession of heat, is 

 called incandescence. 



The more intense the heat is which is imparted to an incan- 

 descent body, the more white will be the light which it emits. 

 When it first becomes luminous, it emits a dusky-red light. The 

 redness becomes brighter as the heat is augmented, until at 

 length, when the heat becomes extremely intense, it emits a white 

 light resembling solar light. 



A bar of iron submitted to the action of a furnace will exhibit a 

 succession of phenomena illustrative of this. 



8. Certain bodies, when surrounded by atmospheric air, being 

 heated to a certain degree, will enter into chemical combination 

 with the oxygen gas which forms one of the constituents of the 

 atmosphere. 



This combination will be attended with a large development of 

 heat, which is accompanied usually by incandescence and name. 



This phenomenon is called combustion, and the bodies which are 

 susceptible of this effect are called combustibles. 



The flame, which is one of the effects of combustion, is gas 

 rendered incandescent by heat. 



The phenomena of combustion and properties of combustibles 

 have been fully explained in our Tract on * Fire." 



9. The degree of sensible heat by which a body is affected, 

 is called its temperature, and the instruments by which the 

 temperature of bodies is indicated and measured are called 

 thermometers and pyrometers ; the latter term being applied to 

 those which are adapted to the measurement of the higher order 

 of temperatures. 



Changes of temperature are indicated and measured by the 

 change of volume which they produce upon bodies very susceptible 

 of dilatation. Such bodies are called thermoscopic bodies. The 

 principal of these are, for thermometers, mercury, alcohol, and 

 air ; and, for pyrometers, the metals, and especially those which 

 are most difficult of fusion. 



L 2 147 



