possibility of appealing for the measurement of temperature to a
phenomenon which is no longer the variation of the elastic force of a
gas, and yet is also connected with the principles of thermodynamics.

This is what Professors Lummer and Pringsheim have shown in a series
of studies which may certainly be reckoned among the greatest
experimental researches of the last few years. They have constructed a
radiator closely resembling the theoretically integral radiator which
a closed isothermal vessel would be, and with only a very small
opening, which allows us to collect from outside the radiations which
are in equilibrium with the interior. This vessel is formed of a
hollow carbon cylinder, heated by a current of high intensity; the
radiations are studied by means of a bolometer, the disposition of
which varies with the nature of the experiments.

It is hardly possible to enter into the details of the method, but the
result sufficiently indicates its importance. It is now possible,
thanks to their researches, to estimate a temperature of 2000 deg. C. to
within about 5 deg. Ten years ago a similar approximation could hardly
have been arrived at for a temperature of 1000 deg. C.

Sec. 6. DERIVED UNITS AND THE MEASURE OF A QUANTITY OF ENERGY

It must be understood that it is only by arbitrary convention that a
dependency is established between a derived unit and the fundamental
units. The laws of numbers in physics are often only laws of
proportion. We transform them into laws of equation, because we
introduce numerical coefficients and choose the units on which they
depend so as to simplify as much as possible the formulas most in use.
A particular speed, for instance, is in reality nothing else but a
speed, and it is only by the peculiar choice of unit that we can say
that it is the space covered during the unit of time. In the same way,
a quantity of electricity is a quantity of electricity; and there is
nothing to prove that, in its essence, it is really reducible to a
function of mass, of length, and of time.

Persons are still to be met with who seem to have some illusions on
this point, and who see in the doctrine of the dimensions of the units
a doctrine of general physics, while it is, to say truth, only a
doctrine of metrology. The knowledge of dimensions is valuable, since
it allows us, for instance, to easily verify the homogeneity of a
formula, but it can in no way give us any information on the actua