Other things being equal, the volume of voice used measures the value that
the mind puts upon the thought. Of course the expression of this value is
modified and characterized by the nature of the thing spoken of. For
example, one would express the value of the ocean with a different quality
from that which would be used in expressing the value of something
exquisitely delicate. All elements of expression modify each other, so
that no mere rule can cover all cases. Volume is not always expressed in
the form of extension of power, but is frequently manifested in the form
of intensity or compressed volume. It is scarcely necessary to explain the
difference between the expression of mere vital power in the voice and
that manifestation of the will which gives the impression of directed
energy. The will determines, and the impetus of the thought is measured
by, the adjustment of volume. Vitality is expressed in radiation; will is
expressed in focus.

The term "volume" may be broadly used to cover the characteristics of the
thing estimated, and hence to include something of that subtle expression
which we call color in the voice. Volume expresses will; color expresses
imagination. For this use of the voice in the special service of
will-power, or propelling force, it is necessary first to test its
freedom. This may be done by taking the humming tone and bringing to bear
upon it a strong pressure of energy. If the tone sharpens under the strain
it is not perfectly focused. If it remains mellow one may venture upon the
next step, which is to practise various vowel sounds and elements of
speech with concentrated energy. The sense of bearing on to the voice, or
endeavoring to push the tone by any pressure whatever, should be
absolutely avoided. Tone support should be carefully regarded. In order to
secure this a correct standing position must be held and the muscles about
the waist and the abdominal muscles must be firm and elastic.