velocity
enables a soft material to cut a harder one. Thus, a wrought iron disc
rotating rapidly, will cut such hard substances as agate or quartz.

RESISTANCE.--All metals offer more or less resistance to the flow of an
electric current. Silver offers the least resistance, and German silver
the greatest. Temperature also affects the flow. It passes more easily
over a cold than a warm conductor.

PERSISTENCE.--All metals on receiving heat, will retain it for a certain
length of time, and will finally cool down to the temperature of the
surrounding atmosphere. Some, like aluminum, retain it for a long time;
others, as iron, will give it off quickly.

CONDUCTIVITY.--All metals will conduct heat and cold, as well as
electricity. If one end of a metal bar is heated, the heat creeps along
to the other end until it has the same temperature throughout. This is
called _equalization_.

If a heated bar is placed in contact with another, the effect is to
increase the temperature of the cold bar and lower that of the warm bar.
This is called _reciprocity_.

MOLECULAR FORCES.--_Molecular_ attraction is a force which acts in such
a way as to bring all the particles of a body together. It acts in
three ways, dependent on the particular conditions which exist.

First: _Cohesion_. This exists between molecules which are of the same
kind, as for instance, iron. Cohesion of the particles is very strong in
solids, much weaker in liquids, and scarcely exists at all between the
particles in gases.

Second: _Adhesion_ is that property which exists between the surfaces of
bodies in contact. If two flat surfaces are pressed together, as for
instance, two perfectly smooth and flat pieces of lead, they will
adhere. If, for instance, oil should be put on the surfaces, before
putting them together, they would adhere so firmly that it would be
difficult to pull them apart.

Third: _Affinity_. This is another peculiarity about materials. Thus,
while cohesion binds together the molecules of water, it is chemical
affinity which unites two elements, like hydrogen and oxygen, of which
water is composed.

POROSITY.--All matter has little hollows or spaces between the
molecules. You know what this is in the case of a sponge, or pumice
stone. Certain metals have the pores so small that it is difficult to
see them except with a very powerful glass. Under great pressure water
can be forced through the pores of metals, as has been done in the case