SECT, xxv.] APPLICATION OF HEAT. 263 



immediately above the evaporating surface, and retards the 

 formation of more ; whereas a strong wind accelerates the 

 process, by carrying off the vapour as soon as it rises, and 

 making way for a succeeding portion of dry air. 



The latent heat of air and of all elastic fluids may be 

 forced out by sudden compression like squeezing water out 

 of sponge. The quantity of heat brought into action in this 

 way is very well illustrated in the experiment of igniting a 

 piece of timber by the sudden compression of air by a piston 

 thrust into a cylinder closed at one end : the development of 

 heat on a stupendous scale is exhibited in lightning, pro- 

 bably produced in part by the violent ^compression of the 

 atmosphere during the passage of the electric fluid. Pro- 

 digious quantities of heat are constantly becoming latent, 

 or are disengaged by the changes of condition to which sub- 

 stances are liable in passing from the solid to the liquid, 

 and from the liquid to the gaseous form, or the contrary, 

 occasioning endless vicissitudes of temperature over the 

 globe. 



There are many other sources of heat, such as combustion, 

 friction, and percussion, all of which are only means of 

 calling a power into evidence which already exists. 



The application of heat to the various branches of the 

 mechanical and chemical arts has, within a few years, effected 

 a greater change in the condition of man than had been ac- 

 complished in any equal period of his existence. Armed 

 by the expansion and condensation of fluids with a power 

 equal to that of the lightning itself, conquering time and 

 space, he flies over plains, and travels on paths cut by human 

 industry even through mountains, with a velocity and smooth- 

 ness more like planetary than terrestrial motion ; he crosses 

 the deep in opposition to wind and tide ; by releasing the 

 strain on the cable, he rides at anchor fearless of the storm ; 

 he makes the elements of air and water the carriers of 

 warmth, not only to banish winter from his home, but to 

 adorn it even during the snow-storm with the blossoms of 



