105. The perispirit, in virtue of its nature, is invisible in its normal state, like a
multitude of fluids which are known to exist, but which we have never seen ; but it can
also, like certain fluids, undergo modifications which render it perceptible to the sight,
sometimes by a kind of condensation, sometimes by a changed arrangement of its
molecules; and it is then that it appears in a vaporous form. What, for want of a better
term, and merely as a comparison, we may term the condensation of the perispirit, gives
to the latter for the time being, all the properties of a solid and tangible body; but the
perispirit, thus condensed, can instantly resume its ethereal and invisible state. We may
understand something of this effect by Comparing the perispirit to aqueous vapour,
which changes from an invisible state to that of mist, becomes liquid or solid, and again
becomes invisible. 'These different states of its perispirit are deter-mined by the will of
the spirit, and do not result from the action of an exterior physical cause, as is the case
in the changes which take place in the state of gases. When a spirit appears, it is
because he puts his perispirit into the necessary condition to render it visible; but the
mere effort of his will does not suffice to this end, for the modification of the perispirit
is effected by its combination with the personal fluid of the medium, which
combination is not always possible; a fact which explains why spirits are not generally
visible. Evidently, therefore, it is not enough that the spirit desires to show himself; it
is not enough that the mortal desires to see him it is necessary that the fluids of the
incarnate and disincarnate spirits should be able to enter into the requisite combination,
that there should be a sort of affinity between them, and, probably, that the emission of
fluid from the mortal should be sufficiently abundant to enable the spirit to effect the
transformation of his perispirit. It is probable, also, that
there are other conditions, of the operation of which we are still in ignorance; and,
moreover, it is necessary that the spirit should have received permission to make
himself visible to a given person; a permission which is not always accorded.
