i88i.] 
Latent Heat . 
589 
There are other instances of great increase and decrease of 
attractive energy to be considered. Thus chemical integra- 
tion produces an immediate and great increase of attractive 
vigour between particles. There is, therefore, a marked in- 
crease in resisting vigour, and much less heat is needed to 
yield unit effects of temperature. The excess heat of the 
compound flows out as aCtive heat energy. It is no longer 
needed by the combining particles, and becomes effective 
elsewhere. In chemical disintegration this effect is reversed. 
There is a marked loss of attractive vigour between the se- 
parating particles. They become greatly individualised, and 
must absorb considerable motive energy from without to 
bring them into temperature equilibrium with surrounding 
matter. Or, in the usual phraseology, in chemical integra- 
tion much latent heat becomes sensible ; in chemical disin- 
tegration much sensible heat becomes latent. 
There remains one other phenomenon of latent heat to be 
considered. We have so far viewed the latency of heat as 
a result of decreased internal resistance, arising from the 
gradual or sudden decrease of attractive energy between 
molecules. But a like effeCt may arise from another cause, 
that of increase or decrease of external pressure. This 
aCtion, while of little effeCt upon solids and liquids, is very 
effective upon gases, and causes marked changes in their 
heat contents. If gases be greatly compressed a consider- 
able volume of their latent heat becomes sensible. If they 
be greatly expanded a considerable volume of their sensible 
heat becomes latent. In the first case their temperature 
markedly rises ; in the second it as markedly falls. The 
result here seen is in strict accordance with the principle 
above enunciated. The compression of a cubic foot of air 
into a cubic inch of space may not necessarily increase the 
vigour of attraction between its molecules, but it must in- 
crease the effective energy of these molecules. The surface 
against which they strike is only i-iqqth its former extent, 
and thus the impacting energy of the molecules is concen- 
trated upon a much smaller space. They need less absolute 
heat to produce the same temperature effeCt as before, and 
their excess motive energy is yielded as sensible heat. In 
the case of expansion the surface upon which the molecules 
acff is increased in extent, and their motive energy must be 
augmented to yield an equal effeCt. It is also possible that 
the results arising in these two cases may be aided by a va- 
riation in the effective vigour of intermolecular attraction, 
this increasing in compressed vapour, and decreasing in ex- 
panded vapour. This last mode of varying the degree of 
