concludes, after a close discussion of the various results obtained,
that this heat is equal to 79.1 calories. An error of almost a calorie
had been committed by several renowned experimenters, 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 (_éclairement_), light (_éclat_), and lighting
(_éclairage_), 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 Expérimentale_