VI
183
ON HARDNESS
From the pressures necessary to produce the first crack
under given circumstances, as well as from the size of this
crack, we get the hardness of the glass. Thus experiments in
which I pressed a hard steel lens against mirror glass gave the
value 130 to 140 kg/mm² for the hardness of the latter.
From the phenomena accompanying the impact of two glass
spheres, I estimated the hardness at 150; whilst a much
larger value, 180 to 200, was deduced from the cracks pro-
duced in pressing together two thin glass bars with natural
surfaces. These differences may in part be due to the defici-
encies of the methods of experimenting (since the same method
gave rise to considerable variations in the various results);
but in part they are undoubtedly caused by want of homogeneity
and by differences in the value of the surface-strength. If
variations as large as the above are found to be the rule, then
of course the numerical results drawn from our theory lose
their meaning; even then the considerations advanced above
afford us an estimate of the value which is to be attributed to
exact measurements of hardness.