liquefy, hydrogen has been the
object of particular research and of really quantitative experiments.
Its properties in a liquid state are now very clearly known. Its
boiling-point, measured with a helium thermometer which has been
compared with thermometers of oxygen and hydrogen, is -252 deg.; its
critical temperature is -241 deg. C.; its critical pressure, 15
atmospheres. It is four times lighter than water, it does not present
any absorption spectrum, and its specific heat is the greatest known.
It is not a conductor of electricity. Solidified at 15 deg. absolute, it
is far from reminding one by its aspect of a metal; it rather
resembles a piece of perfectly pure ice, and Dr Travers attributes to
it a crystalline structure. The last gas which has resisted
liquefaction, helium, has recently been obtained in a liquid state; it
appears to have its boiling-point in the neighbourhood of 6 deg.
absolute.[11]

[Footnote 11: M. Poincare is here in error. Helium has never been
liquefied.--ED.]

Sec. 3. SOLIDS AND LIQUIDS

The interest of the results to which the researches on the continuity
between the liquid and the gaseous states have led is so great, that
numbers of scholars have naturally been induced to inquire whether
something analogous might not be found in the case of liquids and
solids. We might think that a similar continuity ought to be there met
with, that the universal character of the properties of matter forbade
all real discontinuity between two different states, and that, in
truth, the solid was a prolongation of the liquid state.

To discover whether this supposition is correct, it concerns us to
compare the properties of liquids and solids. If we find that all
properties are common to the two states we have the right to believe,
even if they presented themselves in different degrees, that, by a
continuous series of intermediary bodies, the two classes might yet be
connected. If, on the other hand, we discover that there exists in
these two classes some quality of a different nature, we must
necessarily conclude that there is a discontinuity which nothing can
remove.

The distinction established, from the point of view of daily custom,
between solids and liquids, proceeds especially from the difficulty
that we meet with in the one case, and the facility in the other, when
we wish to change their form temporarily or permanently by the action
of mechanical force. This distinction only corresponds, ho