586 
Latent Heat. 
[October, 
own than is possessed by the stick wielded by the strong 
arm. If it fails to receive equal energy from without, it 
must possess greater energy in itself. This is the whole 
secret of latent heat. Molecules are partly individuals and 
partly constituents of a more or less rigid whole. They are 
in incessant motion, and come into very frequent contact 
with each other. By equality of temperature we mean that 
these molecules strike each other with equal energy, and 
resist repulsion with equal energy. But for this equality of 
effeCt it is not necessary that their individual motive energies 
shall always be equal. For if they are parts of a rigid mass 
their strength of resistance comes only partly from them- 
selves, partly from their fellows. But as the mass loses 
rigidity each molecule receives less and less aid from its 
fellows, and has to depend more and more on its individual 
vigour. Each molecule is attached by attraction to its 
neighbours, as if it occupied the centre of an elastic band 
whose ends were firmly fixed. The more rigid such a band, 
the less individual energy would be needed by the molecule 
to give it a unit vigour of resistance. The less rigid the 
band, the more individual energy would be requisite. Two 
molecules of equal weight strike each other with equal speed. 
Their momentums are equal, and they should recede with 
equal energy. But one of them is firmly held by links of 
attraction to surrounding molecules, while the other is but 
weakly held ; therefore the first cannot recede without drag- 
ging a number of others with it, while the drag of the second 
is much less. Thus the receding energy of the first, being 
resisted through its rigid connection with many others, is 
much less than that of the second, which is less resisted. 
The first molecule imparts more energy to the second than 
it receives in return, or, in other words, the temperature 
effeCt of the first exceeds that of the second. The temper- 
ature effects of the two can only be equalised by giving the 
second more motive energy, or increasing its individual mo- 
mentum. And this excess of motive energy is what we 
know as Latent Heat. 
We have the whole mystery here. As attractive energy 
between the molecules diminishes, each of them needs more 
individual motive energy to give it equal resistance to impaCt. 
This applies to all the molecules of a mass, and therefore 
the whole mass needs more heat motion to produce unit 
temperature effects as its internal attractive vigour decreases. 
But the result of heating a solid mass is to decrease the 
vigour of attraction between its molecules. It expands with 
increase of temperature, and becomes less rigid. As a 
