ce of the
weight, the part which presses on the ice, consists of a light,
brass curtain ring. This can be detached. Its mass is only 61/2
grams, the curtain ring being, in fact, hollow and made of very
thin metal. We have, therefore, in it a very small weight which
presents exactly the same surface beneath as did the weight of
250 grams. You see, now, that this light weight will not slip on
ice at 5 or 6 degrees of slope, but first does so at about io
degrees.

This is a very important experiment as regards our present
inquiry. Ice appears to possess more than one angle of friction
according as a heavy or a light weight is used to press upon it.
We will make the same experiment with the plate of glass. You see
that there is little or no difference in the angle of friction of
brass on glass when we press the surfaces together with a heavy
or with a light weight. The light weight requires the same slope
of 121/2 degrees to make it slip.

This last result is in accordance with the laws of friction. We
say that when solid presses on solid, for each pair of substances
pressed together there is a constant ratio between the force
required to keep one in motion over the other, and the force
pressing the solids together. This ratio is called"the
coefficient of friction."The coefficient is, in fact, constant or
approximately

262

so. I can determine the coefficient of friction from the angle of
friction by taking the tangent of the angle. The tangent of the
angle of friction is the coefficient of friction. If, then, the
coefficient is constant, so, of course, must the angle of
friction be constant. We have seen that it is so in the case of
metal on glass, but not so in the case of metal on ice. This
curious result shows that there is something abnormal about the
slipperiness of ice.

The experiments we have hitherto made are open to the reproach
that the surface of the ice is probably damp owing to the warmth
of the air in contact with it. I have here a means of dealing
with a surface of cold, dry ice. This shallow copper tank about
18 inches (45 cms.) long, and 4 inches (10 cms.) wide, is filled
with a freezing 'mixture circulated through it from a larger
vessel containing ice melting in hydrochloric acid at a
temperature of about -18 deg. C. This keeps the tank below the
melting point of ice. The upper surface of the tank is provided
with raised edges so that it can be flooded with water. The water
is now frozen and