INTRODUCTION
xix
necessarily introduced an element of uncertainty depending
upon the pressure exerted: a method of ascertaining its
magnitude with certainty was wanting. Now the question
could be answered definitely and at once. In technical circles
equal interest was exhibited, and this induced Hertz to extend
the investigation further and to allow it to be published not
only in Borchardt's Journal (V. in this volume) but also in a
technical journal, with a supplement on Hardness (VI.)
About this he writes to his parents as follows:-
:
9th May 1882.
I have been writing a great deal lately; for I have rewritten
the investigation once more for a technical journal in compliance
with suggestions which reached me from various directions.
I have also added a chapter on the hardness of bodies, and hope to
lecture on this to the Physical Society on Friday. I have had
some fun out of this too. For hardness is a property of bodies of
which scientific men have as clear, i.e. as vague, a conception as
the man in the street. Now as I went on working it became quite
clear to me what hardness really was.
But I felt that it was
not in itself a property of sufficient importance to make it
worth while writing specially about it; nor was such a subject,
which would necessarily have to be treated at some length, quite
suitable for a purely mathematical journal. In a technical
journal, however, I thought I might well write something about
the matter. So I went to look round the library of the Gewerbe-
akademie, and see what was known about hardness. And I found
that there really was a book written on it in 1867 by a Frenchman.
It contained a full account of earlier attempts to define hardness
clearly, and to measure it in a rational way, and of many experi-
ments made by the writer himself with the same object, interspersed
with assurances as to the importance of the subject. Altogether
it must have involved a considerable amount of work, which was
labour lost-so I think, and he partly admits it because there
was no right understanding at the bottom of it, and the measure-
ments were made without knowing what had to be measured. So
I concluded that now I might with a quiet conscience make my
paper a few pages longer; and since this I have naturally had
much more pleasure than before in writing it out.
Whilst these problems on elasticity were engaging his
attention Hertz was also busy with the researches on evapora-
tion (VIII. and IX. in this volume) and the second investi-
gation on the Kinetic Energy of Electricity in Motion (IV.)