Page:Elementary Text-book of Physics (Anthony, 1897).djvu/225

§ 193] '''193. Dulong and Petit's Law. Atomic Heat.'''— In their study of the specific heats of a number of chemical elements which are solid at ordinary temperatures, Dulong and Petit found that the product of the specific heat by the atomic weight of the element was approximately a constant quantity. Further researches, especially those of Kopp, have confirmed this statement as a general law for all solid elements. The constant number to which the product of the specific heat and the atomic weight approximates is ordinarily given as 6.4 when the specific heat is measured in calories, though this is probably a little too high. The deviations from this number presented by different elements are rather large, amounting in many cases to as much as 5 per cent.

If masses of the different elements be taken which are proportional to their atomic weights, these masses will contain the same numbers of atoms. The heat required to raise one of these masses one degree in temperature is therefore the same for all such substances. This statement is of course true only within the limits of accuracy with which the, different substances conform to Dulong and Petit's law. The experiments of P. Neumann and Regnault showed that a similar law applies to compounds of solids which are of the same chemical constitution; that is, which contain the same number of atoms in the molecule. For all such bodies the product of the specific heat and the molecular weight is a constant; this constant is different for the different classes of substances—that is, for those substances which have different numbers of atoms in the molecule. But if the constant obtained for each class of substances be divided by the number of atoms in the molecule of that class, the quotient is approximately the same constant, 6.4, as that obtained for the elements. By applying this law to compounds in which one of the elements is a substance, like hydrogen, which cannot be examined directly in the solid state, the atomic heat of that substance may be calculated. It is found that the atomic heats of certain substances, notably hydrogen, carbon, oxygen, nitrogen, and silicium, deviate very widely from the constant with which the other atomic heats approximately agree.