nked to
the sequence of our states of consciousness does not play a
considerable part.

Ancestral habits and a very early tradition have led us to preserve,
as the unit of time, a unit connected with the earth's movement; and
the unit to-day adopted is, as we know, the sexagesimal second of mean
time. This magnitude, thus defined by the conditions of a natural
motion which may itself be modified, does not seem to offer all the
guarantees desirable from the point of view of invariability. It is
certain that all the friction exercised on the earth--by the tides,
for instance--must slowly lengthen the duration of the day, and must
influence the movement of the earth round the sun. Such influence is
certainly very slight, but it nevertheless gives an unfortunately
arbitrary character to the unit adopted.

We might have taken as the standard of time the duration of another
natural phenomenon, which appears to be always reproduced under
identical conditions; the duration, for instance, of a given luminous
vibration. But the experimental difficulties of evaluation with such a
unit of the times which ordinarily have to be considered, would be so
great that such a reform in practice cannot be hoped for. It should,
moreover, be remarked that the duration of a vibration may itself be
influenced by external circumstances, among which are the variations
of the magnetic field in which its source is placed. It could not,
therefore, be strictly considered as independent of the earth; and the
theoretical advantage which might be expected from this alteration
would be somewhat illusory.

Perhaps in the future recourse may be had to very different phenomena.
Thus Curie pointed out that if the air inside a glass tube has been
rendered radioactive by a solution of radium, the tube may be sealed
up, and it will then be noted that the radiation of its walls
diminishes with time, in accordance with an exponential law. The
constant of time derived by this phenomenon remains the same whatever
the nature and dimensions of the walls of the tube or the temperature
may be, and time might thus be denned independently of all the other
units.

We might also, as M. Lippmann has suggested in an extremely ingenious
way, decide to obtain measures of time which can be considered as
absolute because they are determined by parameters of another nature
than that of the magnitude to be measured. Such experiments are made
possible by the phenomena of gravi