CHAMBERS'S INFORMATION FOR THE PEOPLE. 



when this heat seeks to escape by radiation, the 

 glass refuses to retransmit it. 



LATENT HEAT. 



\Yhen a solid body, such as ice, is watched 

 whilst melting, a large quantity of heat is observed 

 to enter it without raising its temperature in the 

 slightest degree. This heat which enters the 

 body serves only to melt or liquefy it, without 

 rendering the liquid the least hotter than the solid 

 was which yielded it The water which flows 

 from the melting ice is no warmer than the ice. 

 The heat which thus renders a body' liquid with- 

 out warming it, is called latent or insensible heat, 

 because it does not affect our sensations, and does 

 not raise the thermometer. The fact of heat 

 becoming latent, is most decisively demonstrated 

 by mixing a certain quantity say an ounce by 

 weight of ice, or, still better, from its state of 

 division, of snow at 32, with an ounce of water 

 at 172. The result will be found to be, that the 

 snow is all melted, and two ounces of water are 

 procured at the temperature of 32. The hot 

 water in cooling from 172 to 32, has lost 140 of 

 heat, which changes the snow into water, but does 

 not raise its temperature above that originally 

 possessed by the snow. 



What we have illustrated here with ice, holds 

 good for all solids. Each one of them renders 

 latent a certain quantity of heat in becoming 

 liquid, and retains that heat so long as it remains 

 liquid ; when the liquid solidifies, it is again given 

 up. Thus, when water freezes, the 140 of latent 

 heat all abandon it, and manifest themselves as 

 sensible heat. It is this necessity of absorbing 

 such a quantity of heat, and getting rid of it again, 

 that makes the processes of thawing and freezing 

 go on so slowly. 



In evaporating a liquid, a similar disappearance 

 of heat takes place. In boiling off a pound of 

 water, or converting it into vapour, it can be 

 shewn by experiment that as much heat is 

 absorbed as would have raised its temperature 

 about 1000, if it had not gone off in steam. Yet 

 the water rises no higher than 212, however hot 

 the fire is, and the steam is of the very same 

 temperature as the water it rises from. Thus 

 1000 have disappeared or become latent in the 

 steam ; and before the steam can be condensed 

 into water again, all this heat must be given out. 

 See STEAM-ENGINE. 



The same is true of the vapour that rises slowly 

 and silently from water at temperatures below 

 boiling (see METEOROLOGY). This absorption 

 of latent heat is the cause of the cold which 

 always accompanies evaporation. By placing 

 water in a shallow vessel under the receiver of an 

 air-pump, and withdrawing the vapour as fast as 

 it rises, evaporation may be made to go on so 

 rapidly that the water freezes. 



Heat which thus becomes latent, is not lost ; it 

 has done work in producing a change of state in 



the substance in loosening the atoms from the 

 rigid bond of cohesion, and remains a fund of 

 potential energy, like a bent bow, ready to become 

 heat again, or be converted into mechanical power, 

 as in the steam-engine. This is another instance 

 of the conservation of energy. 



SOURCES OF HEAT. 



Next to the sun (see ASTRONOMY), chemical 

 combinations are the chief sources of heat When 

 two substances unite chemically, their temper- 

 ature is almost always raised. When the heat is 

 evolved so rapidly as to render the substances 

 luminous which most substances become when 

 heated to a certain degree the process is called 

 combustion (see CHEMISTRY). Fire is a solid 

 rendered luminous or incandescent by combus- 

 tion ; flame is gas at a white heat. 



Animal heat has the same source as the heat of 

 a fire or of a candle ; it arises from a species of 

 combustion. The oxygen taken into the body by 

 the lungs unites with the carbon and hydrogen of 

 the waste parts of the blood and solids, and con- 

 verts them into carbonic acid and vapour of water 

 burns them, in short, and thus produces heat 

 (See PHYSIOLOGY.) 



Heat can also be produced by mechanical 

 means, such as compression, percussion, and 

 friction. A piece of iron may be rendered hot by 

 hammering ; and axles of carriages often ignite 

 from friction. It is found that the amount of heat 

 thus produced is always in proportion to the 

 mechanical energy expended in the process. 



MECHANICAL EQUIVALENT OF HEAT. 



The exact relation of the heat to the work that pro- 

 duces it is expressed in what is called the mechanical 

 equivalent of heat. A unit of work is the amount 

 expended in raising a pound-weight to the height of 

 a foot, and 10 pounds raised I foot, or i pound 

 raised 10 feet, makes 10 units of work, or 10 foot- 

 pounds. Now, it has been determined by accu- 

 rate experiments that 772 foot-pounds of work pro- 

 duce heat sufficient to raise a pound of water one 

 degree in temperature ; and 772 foot-pounds con- 

 stitute the mechanical equivalent of heat. Another 

 way of stating the same thing is to say, that a 

 pound-weight falling from a height of 772 feet 

 against the earth, generates heat sufficient to raise a 

 pound of water one degree ; or conversely, that the 

 heat that raises a pound of water one degree in 

 temperature, would, if it could be all used mechan- 

 ically, raise 772 pounds a foot high. In this way 

 it has been calculated that if our earth were 

 ' to strike against a target strong enough to stop 

 its motion . . . the amount of heat thus developed 

 would be equal to that derived from the combus- 

 tion of fourteen globes of coal each equal to the 

 earth in magnitude. And if, after the stoppage of 

 its motion, the earth should fall into the sun, as it 

 assuredly would, the amount of heat generated by 

 the blow would be equal to that developed by the 

 combustion of 5600 worlds of solid carbon.' 



