skins of two liquids may be either a source of vexation or, if we
know how to make use of it, an advantage. If you spill grease on your
coat you can take it out very well with benzine. Now if you apply
benzine to the grease, and then apply fresh benzine to that already
there, you have this result--there is then greasy benzine on the coat to
which you apply fresh benzine. It so happens that greasy benzine has a
stronger skin than pure benzine. The greasy benzine therefore plays at
tug-of-war with pure benzine, and being stronger wins and runs away in
all directions, and the more you apply benzine the more the greasy
benzine runs away carrying the grease with it. But if you follow the
directions on the bottle, and first make a ring of clean benzine round
the grease-spot, and then apply benzine to the grease, you then have the
greasy benzine running away from the pure benzine ring and heaping
itself together in the middle, and escaping into the fresh rag that you
apply, so that the grease is all of it removed.

There is a difference again between hot and cold grease, as you may see,
when you get home, if you watch a common candle burning. Close to the
flame the grease is hotter than it is near the outside. It has therefore
a weaker skin, and so a perpetual circulation is kept up, and the grease
runs out on the surface and back again below, carrying little specks of
dust which make this movement visible, and making the candle burn
regularly.

You probably know how to take out grease-stains with a hot poker and
blotting-paper. Here again the same kind of action is going on.

A piece of lighted camphor floating in water is another example of
movement set up by differences in the strength of the skin of water
owing to the action of the camphor.

I will give only one more example.

If you are painting in water-colours on greasy paper or certain shiny
surfaces the paint will not lie smoothly on the paper, but runs together
in the well-known way; a very little ox-gall, however, makes it lie
perfectly, because ox-gall so reduces the strength of the skin of water
that it will wet surfaces that pure water will not wet. This reduction
of the surface tension you can see if I use the same wire frame a third
time. The ether has now evaporated, and I can again make it rest against
the surface of the water, but very soon after I touch the water with a
brush containing ox-gall the frame jumps up as suddenly as before.

It is quite un