CHAPTERS FOR STUDENTS. 
865 
from the water b, that it freezes. The heat which is taken up in this manner, 
when a body changes from solid to liquid or from liquid to gas, is called heat 
of liquefaction, heat of vaporization, or frequently ‘latent heat.’ 
22. The heat which becomes latent in a liquid formed by the melting of a 
solid, is given out, undiminished in amount, when it reverts to the solid state. 
The heat which becomes latent in the vapour produced by the volatilization of 
a liquid, is also given out when the vapour condenses to the liquid form again. 
The heat which thus enters into combination with the body produces no effect 
upon its temperature, it merely changes its physical state; therefore, ice at 32° 
+ latent heat = water at 32° • also water at 212° +latent heat = steam at 
212 °. 
23. The foregoing examples and considerations thereupon will explain many 
commonly-occurring phenomena. When pounded ice is mixed with salt, the 
whole liquefies from the mutual chemical attraction of the salt and water; in¬ 
tense cold is the result. If ether is allowed to evaporate from the hand, a sen¬ 
sation of cold is produced ; the ether in undergoing the change of condition 
flies off with heat from the hand. 
If the hand be held in the steam issuing from a kettle, scalding results. The 
steam in condensing from the vaporous to liquid state gives out all the heat 
w hich it had held latent, and this heat is sufficient to cause a burn. 
If the experiment be made of mixing together cold saturated solutions of sul¬ 
phate of magnesia and carbonate of soda, the quantity of precipitate formed is 
so great that the whole becomes a nearly solid mass ; at the same time it be¬ 
comes perceptibly hot ; at least the greater part of the heat so becoming sensible 
is the result of the change from liquid to solid. When ice is caused to melt by 
the application of heat, the water formed retains precisely the same temperature, 
32’ F., until the last of the solid W'ater has been converted into liquid. The 
heat as it goes in is totally consumed in effecting the change of state. Similarly, 
water boiling in a metallic vessel retains a uniform and not increasing tempe¬ 
rature ; all the heat supplied to it from without being employed in changing it 
into steam. 
24. Conversion of a liquid into vapour may take place in two ways; either 
by “ evaporation ” from the surface only, or by “ebullition,” or boiling. A 
liquid is said to boil when bubbles of vapour are formed in the body of the 
liquid and rising to the surface burst there. 
25. These bubbles can only be formed when the heat applied is sufficient to 
give the vapour an expansive power (sometimes called tension) sufficient to over¬ 
come the weight of the atmosphere superincumbent upon the surface of the 
liquid. In the ordinary state of the atmosphere, this pressure is such that one 
square inch of surface is pressed upon by a pressure equal to about 15 pounds. 
Under this pressure water boils at 212° F., or 100° C. 
If a small box containing some water w T ere constructed with a stopper or 
valve, having an area of 1 square inch, and upon this a weight of 15 pounds 
were placed, and the apparatus then placed in a vacuum, the vapour would just 
lift the valve when the w r ater had been heated to 212° F. 
If instead of working the apparatus in a vacuum it were heated in the air, 
the addition of the atmospheric pressure would render it necessary to raise the 
temperature to about 250° F. in order to lift the valve, which would now be 
under a weight of about 30 pounds. Pressure, therefore, raises the boiling- 
point. 
26. On the other hand, removal of pressure, atmospheric or other, lowers the 
boiling-point of a liquid. On this account evaporations and distillations are 
often conducted in a partial vacuum, in cases wdiere the application of much 
heat is undesirable, as in making medicinal extracts. 
A small apparatus for distillation may easily be constructed with the aid of 
