i88ij 
Latent Heat. 
5«7 
consequence its specific heat increases, or a portion of its 
heat becomes latent, or, in the terms of this hypothesis, 
each molecule, and therefore the mass as a whole, needs 
more individual heat motion to give it a unit resistance to 
exterior heat impacSt. 
When solids are far from their melting-point this effedt is 
slight, as they decrease very slowly in rigidity. Thus it is 
inappreciable in platinum at ordinary temperatures, since 
this metal is then very far from its melting-point. But as 
solids approach their melting-points the increase in specific 
heat becomes declared. It is especially marked in those 
solids which soften before they melt, and it thus shows its 
relation to decreased rigidity. On melting there is a very 
considerable increase of specific heat, and particularly in 
those substances which continue rigid to the moment of 
melting. In passing from the last degree of solidity to the 
first degree of liquidity the specific heat suddenly becomes 
very great. But the rigidity very greatly decreases, so that 
the two effedts continue in accordance. The molecules of 
the liquid have to depend far more on their individual energy 
than do those of the solid, and are thus obliged to absorb a 
considerable volume of motive energy to put them into tem- 
perature equilibrium with the solid. 
The same rule holds with liquids as with solids. As they 
slowly expand with heat their internal attractive vigour 
decreases, and their specific heat increases. On changing 
to the gaseous state there is another sudden decrease of 
rigidity. The attractive control of molecule upon molecule 
very markedly decreases, and there is a sudden and enor- 
mous increase of specific heat in the degree of temperature 
leading from the liquid to the gas. The molecules have 
become strongly individualised, and need a great increase in 
motive energy to place them in temperature equilibrium with 
solids and liquids. The same principle holds good with 
gases. As their temperature rises above the point of vapor- 
isation the attractive energy between their molecules probably 
continues to decrease, and their specific heat increases. 
When, however, their temperature is considerably above the 
point of vaporisation, the specific heat becomes constant. 
It is presumable that their rigidity has also become constant, 
as in solids when far below their melting-point. 
This is latent heat in its ordinary acceptation. There is 
no real loss of heat energy, and no transfer of motive vigour 
from the centrifugal to the centripetal field. All the heat 
energy received continues to be energy of molecular impact, 
and it increases in relative amount to the exact degree that 
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