ths of a
millimetre.

Small as is the above amount of oil, the camphor test is a comparatively
coarse one. Conditions of a contaminated surface may easily be
distinguished, upon all of which camphor fragments spin vigorously.
Thus, a shallow tin vessel, such as the lid of a biscuit box, may be
levelled and filled with tap-water through a rubber hose. Upon the
surface of the water a little sulphur is dusted. An application of the
finger for 20 or 30 seconds to the under surface of the vessel will then
generate enough heat to lower appreciably the surface-tension, as is
evidenced by the opening out of the dust and the formation of a bare
spot perhaps 1-1/2 in. in diameter. When, however, the surface is but
very slightly greased, a spot can no longer be cleared by the warmth of
the finger, or even of a spirit lamp, held underneath. And yet the
greasing may be so slight that camphor fragments move with apparently
unabated vigour.

The varying degrees of contamination to which a water surface is subject
are the cause of many curious phenomena. Among these is the _superficial
viscosity_ of Plateau. In his experiments a long compass needle is
mounted so as to swing in the surface of the liquid under investigation.
The cases of ordinary clean water and alcohol are strongly contrasted,
the motion of the needle upon the former being comparatively sluggish.
Moreover, a different behaviour is observed when the surfaces are
slightly dusted over. In the case of water the whole of the surface in
front of the needle moves with it, while on the other hand the dust
floating on alcohol is scarcely disturbed until the needle actually
strikes it. Plateau attributed these differences to a special quality of
the liquids, named by him "superficial viscosity." It has been proved,
however, that the question is one of contamination, and that a water
surface may be prepared so as to behave in the same manner as alcohol.

Another consequence of the tendency of a moderate contamination to
distribute itself uniformly is the calming effect of oil, investigated
by B. Franklin. On pure water the propagation of waves would be attended
by temporary extensions and contractions of the surface, but these, as
was shown by O. Reynolds, are resisted when the surface is contaminated.

Indeed the possibility of the continued existence of films, such as
constitute foam, depends upon the properties now under consideration.
If, as is sometimes stated, the tensio