" Indeed, the exact nature of the lagging matters comparatively
little, because the active substance in retaining the heat in the
acetylene generator or the steam-pipe is the air entangled in the pores
of the lagging; and therefore the value of any particular material
depends mainly on its exhibiting a high degree of porosity. The idea of
fitting a water jacket round an acetylene generator is not altogether
good, but it may be greatly improved upon by putting into the jacket a
strong solution of some cheap saline body which has the property of
separating from its aqueous solution in the form of crystals containing
water of crystallisation, and of evolving much heat in so separating.
This method of storing much heat in a small space where a fire cannot be
lighted is in common use on some railways, where passengers' foot-warmers
are filled with a strong solution of sodium acetate. When sodium acetate
is dissolved in water it manifestly exists in the liquid state, and it is
presumably present in its anhydrous condition (i.e., not combined with
water of crystallisation). The common crystals are solid, and contain 3
molecules of water of crystallisation--also clearly in the solid state.
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