ces and effects, as well as its want of power on oxygen and
hydrogen, are the consequences, and the indications, of a soiled surface.

634. I found also that platina plates which had been cleaned perfectly soon
became soiled by mere exposure to the air; for after twenty-four hours they
no longer moistened freely with water, but the fluid ran up into portions,
leaving part of the surface bare, whilst other plates which had been
retained in water for the same time, when they were dried (580.) did
moisten, and gave the other indications of a clean surface.

635. Nor was this the case with platina or metals only, but also with
earthy bodies, Rock crystal and obsidian would not wet freely upon the
surface, but being moistened with strong oil of vitriol, then washed, and
left in distilled water to remove all the acid, they did freely become
moistened, whether they were previously dry or whether they were left wet;
but being dried and left exposed to the air for twenty-four hours, their
surface became so soiled that water would not then adhere freely to it, but
ran up into partial portions. Wiping with a cloth (even the cleanest) was
still worse than exposure to air; the surface either of the minerals or
metals immediately became as if it were slightly greasy. The floating upon
water of small particles of metals under ordinary circumstances is a
consequence of this kind of soiled surface. The extreme difficulty of
cleaning the surface of mercury when it has once been soiled or greased, is
due to the same cause.

636. The same reasons explain why the power of the platina plates in some
circumstances soon disappear, and especially upon use: MM. Dulong and
Thenard have observed the same effect with the spongy metal[A], as indeed
have all those who have used Doebereiner's instantaneous light machines. If
left in the air, if put into ordinary distilled water, if made to act upon
ordinary oxygen and hydrogen, they can still find in all these cases _that_
minute portion of impurity which, when once in contact with the surface of
the platina, is retained there, and is sufficient to prevent its full
action upon oxygen and hydrogen at common temperatures: a slight elevation
of temperature is again sufficient to compensate this effect, and cause
combination.

[A] Annales de Chimie, tom. xxiv. p. 386.

637. No state of a solid body can be conceived more favourable for the
production of the effect than that which is possessed by plati