South Bend news-times. (South Bend, Ind.) 1913-1938, 1918-04-30

diamond or a piece or steer -
friend says his diamond ring was
crushed to powder between two nag
stones, and yet 1 have heard that a
diamond run over by a train VIII
make d dent or groove in the rail
and. not be damaged. w
There is no doubt a great deal of
confusion concerning the meaning
Of the physical properties" of sub
stances. The writer of the letter
above is by no means alone in sup
posing that hardness" is identical
with or else directly proportional 10
"tenacity." As a matter of fact a
substance may be both hard and
fragile or easily broken into frag-
ments. The hardness of a body is
determined by its ability 10 resist
scratching or abrasion by another,
as. for instance, when rubbed with
file. The test is a force applied TO
the surface of the body, and acting
upon a relatively small area. The
quality called hardness seems to de-
pend upon the crystalline structure
OF the substance, and il is manifest
ed by the resistance of the mole
cules composing the ultimate crystal
particles TO a force tending to tear
them apart. Ii often happens that
a crystal can be more easily scratch
ed on one of its faces than on an
other The diamond exhibits the
property of hardness in a higher de-
gree than any other mineral. and
will scratch any other. but this does
not imply that it cannot be broken
or crushed into fragments when the
stresses have, so to speak, sufficient
leverage to overcome the cohesion.
Resistance TO these larger forces
is furnished by the property of ten
acity, which has been defined as a
measure Of the stress required for
the rupture OF a mass of a given
cross-section. Struck by a sledge-
hammer or by the wheel of a rail-
road train. the diamond, although il
might make an indentation in the
iron or steel. would nevertheless
yield, as a whole. TO the tremendous
energy e the blow. It's very rigid
ity be a cause Of its disrup-
ton
But a piece Of iron. although far
less hard than the diamond. would
simply be fattened by the same
blow Steel being harder than
iron. is proportionately more rigid,
and. under the same circumstances.
would -, badly be broken, though
not crushed in the manner Of the
diamond
Perhaps the difference between
hardness and tenacity can be illus
trated in the following way. Sup
pose you take u sticks and put
their ends together in such man
ner that they Will form the edges of
a cube Then, in order 10 hold them
together. run tightly stretched
strings from corner 10 corner and
criss-cross. so that the entire struc-
ture is made exceedingly rigid by
virture OF the stresses that hold its
parts pressed together. If now you
push upon II at any point it will
strongly resist being bent, distorted
or indented: in other words il viii
exhibit a property akin 20 hardness.
But if YOU hit 1t A smashing blow
with a force exceeding the tensile
strength Of the strings il Will all go
Z 20 pieces at once. Take next the
same sticks. arranged in the same
form. except that instead OF keep
/ Ing them under the stress OF cross
binding strings YOU simply tie them
| together NJ each OF the corners.
Now your cube will be much less
rigid than before. but II viii have a
kind Of adjustable Conference that
| was lacking in the other case. You