What thermal properties of substances could not be explained by the classical molecular kinetic theory?

In the popular history of quantum mechanics, much attention is paid to the so-called “ultraviolet catastrophe”.
At the end of the 20th century, scientists, building their theories on the basis of classical physics, concluded that the power of electromagnetic radiation from a black body – which can be thought of as a warm and completely absorbing light object – should become infinite as the wavelength decreases from visible to ultraviolet. spectrum.
But, obviously, this is not so, and in 1900 Max Planck tried to resolve this “catastrophe”, assuming that vibrating atoms of a black body can emit energy only in discrete portions – quanta – proportional to the frequency of their vibrations. By letter he designated the coefficient of proportionality, called Planck’s constant.
It is still not clear what meaning Planck himself put into the concept of “quanta”, but, apparently, for a long time he refused to recognize the “reality” of his assumption, considering them just a convenient mathematical construction.
Later it turned out that they really help to resolve the ultraviolet catastrophe, imposing restrictions on how the atoms of a dark body, vibrating at a high frequency, can emit energy. For this idea in 1918, Planck received the Nobel Prize.

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