gas thermometer. It becomes impossible to guarantee the reservoir
remaining sufficiently impermeable, and all security disappears,
notwithstanding the use of recipients very superior to those of former
times, such as those lately devised by the physicists of the
_Reichansalt_. This difficulty is obviated by using other methods,
such as the employment of thermo-electric couples, such as the very
convenient couple of M. le Chatelier; but the graduation of these
instruments can only be effected at the cost of a rather bold
extrapolation.

M.D. Berthelot has pointed out and experimented with a very
interesting process, founded on the measurement by the phenomena of
interference of the refractive index of a column of air subjected to
the temperature it is desired to measure. It appears admissible that
even at the highest temperatures the variation of the power of
refraction is strictly proportional to that of the density, for this
proportion is exactly verified so long as it is possible to check it
precisely. We can thus, by a method which offers the great advantage
of being independent of the power and dimension of the envelopes
employed--since the length of the column of air considered alone
enters into the calculation--obtain results equivalent to those given
by the ordinary air thermometer.

Another method, very old in principle, has also lately acquired great
importance. For a long time we sought to estimate the temperature of a
body by studying its radiation, but we did not know any positive
relation between this radiation and the temperature, and we had no
good experimental method of estimation, but had recourse to purely
empirical formulas and the use of apparatus of little precision. Now,
however, many physicists, continuing the classic researches of