ers, and it will be
seen that in certain points the art of measurement may still be
largely perfected.

To the unit of energy might be immediately attached other units. For
instance, radiation being nothing but a flux of energy, we could, in
order to establish photometric units, divide the normal spectrum into
bands of a given width, and measure the power of each for the unit of
radiating surface.

But, notwithstanding some recent researches on this question, we
cannot yet consider the distribution of energy in the spectrum as
perfectly known. If we adopt the excellent habit which exists in some
researches of expressing radiating energy in ergs, it is still
customary to bring the radiations to a standard giving, by its
constitution alone, the unit of one particular radiation. In
particular, the definitions are still adhered to which were adopted as
the result of the researches of M. Violle on the radiation of fused
platinum at the temperature of solidification; and most physicists
utilize in the ordinary methods of photometry the clearly defined
notions of M. Blondel as to the luminous intensity of flux,
illumination (_eclairement_), light (_eclat_), and lighting
(_eclairage_), with the corresponding units, decimal candle, _lumen_,
_lux_, carcel lamp, candle per square centimetre, and _lumen_-hour.[4]

[Footnote 4: These are the magnitudes and units adopted at the
International Congress of Electricians in 1904. For their definition
and explanation, see Demanet, _Notes de Physique Experimentale_
(Louvain, 1905), t. iv. p. 8.--ED.]

Sec. 7. MEASURE OF CERTAIN PHYSICAL CONSTANTS

The progress of metrology has led, as a consequence, to corresponding
progress in nearly all physical measurements, and particularly in the
measure of natural constants. Among these, the constant of gravitation
occupies a position quite apart from the importance and simplicity of
the physical law which defines it, as well as by its generality. Two
material particles are mutually attracted to each other by a force
directly proportional to the product of their mass, and inversely
proportional to the square of the distance between them. The
coefficient of proportion is determined when once the units are
chosen, and as soon as we know the numerical values of this force, of
the two masses, and of their distance. But when we wish to make
laboratory experiments serious difficulties appear, owing to the
weakness of the attraction between masses o