PYRONOMICS. 91 
although some bodies before fusion experience chemical changes. The 
melting point of a body, or the temperature at which it becomes liquid, is 
invariable for one and the same body ; as also, with certain restrictions, is 
the boiling point, or the temperature at which a liquid begins to vaporize. 
During liquefaction the temperature does not alter, however great a degree 
of heat may be applied; the excess of heat, therefore, becomes latent. 
The opposite to melting in a body is its solidification, or the transition of 
a body from the liquid to the solid state. This generally takes place at the 
same temperature of a body as melting, all the combined heat being given 
out. We may be convinced of this by causing water to boil in a glass tube, 
and, when this is filled with steam, melting it together at the open end. If, 
now, the tube be cooled to about 15°F., the water will remain liquid ; at the 
least agitation, however, it will become converted into ice, and a thermo- 
meter placed on the tube will ascend immediately from 15°F. to 32°F. As 
much heat, formerly latent in the water, will therefore be set free as sufficed 
to elevate its temperature 17° F. 
The solidification of bodies takes place in fil went forms, according to the 
circumstances. If it be carried on slowly, a crystallization renee 
of each body takes place ; if the cooling or solidification be accelerated, the 
particles have not time to arrange themselves properly, and irregular, 
confused formations are produced. 
c. The Formation of Vapor. 
If a fluid be in contact with the air, its amount gradually decreases by 
evaporation, or its conversion into vapor. The Torricellian vacuum is best 
adapted for exhibiting the laws of vaporization. In a broad vessel, VV’ 
( pl. 19, fig. 17), place three barometer tubes close to each other, the height 
of the mercury being the same in all. If some water be introduced into 
one of these tubes, as 0’, it will rise to the top, and the mercury will be sensibly 
depressed. This can only be produced by the giving off of a vapor which 
exerts an expansive force ljke the gases. ‘The depression of the mercury 
gives the measure of the tension of the vapor. If some other fluid, as sul- 
phuric ether, be introduced into the third tube, 6’’, there will be observed 
a much greater depression of the mercury, owing to the tension of 
ether vapor being much greater at the same temperature than that of 
water. 
The elasticity or tension of vapor is increased by compression, just like 
air; there is, however, a certain limit or maximum of compression, above 
which the vapor becomes converted into a liquid. This maximum varies 
with the temperature, increasing with its increase. In this circumstance is 
a characteristic difference between vapors and gases. Suppose, in the appa- 
ratus, fig. 23, the upper barometer tube be filled for a few inches with 
mercury from which all air has been removed by boiling, and the rest with 
ether ; now let the tube be inverted and immersed in the vessel cn, and the 
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