Page:Zur Dynamik bewegter Systeme.djvu/25

 thermal function R0 with temperature, volume and chemical composition. For according to equation (48), the inertial mass of a body changes due to thermal input and release, and the increase of mass is always equal to the amount of heat absorbed in an isobar change of the body from the outside, divided by the square of the velocity of light in vacuum. It is particularly noteworthy, that the theorem not only applies to reversible processes, but in general also applies to any irreversible change of state; for the relation between the thermal function R and the heat supplied from the outside is based directly upon the first law of thermodynamics. Because the quantity is of the order of c², the mass variation caused by the simple heating or cooling of a body is so minimal, of course, that it is likely to escape forever any direct measurement. A stronger influence would be expected by consideration of chemical enthalpy changes, although even here the effect is unlikely to be measured.

Let us calculate, for example, the decrease in mass of 1½ moles oxygen-hydrogen (H2 + ½O2 = 18 gr), condensed at atmospheric pressure and room temperature to 1 mol of liquid water. This is equivalent to the heat in CGS units:

r = 68400 · 419 · 105= 2.87 · 1012

Consequently, the decrease in mass: $$\frac{r}{c^{2}}gr=3.2\cdot10^{-6}$$ mgr, is still a vanishingly small quantity.

§ 18. According to the theory developed here, we therefore have to imagine an energy store in the interior of each body, whose amount is so enormous that the usually observed heating and cooling processes, and even quite deep invasive chemical transformations associated with considerable heat effects, changes it by only an imperceptible fraction. This is valid down to the lowest attainable temperatures: for both the specific heat of a body as well as the