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 there is still a condition under which it comes into direct thermodynamic efficiency: that is the occurrence of a change or destruction of the chemical atoms; for in this case some latent energy has to be released in accordance with the energy principle. Although the prospect of the realization of such a radical operation appeared to be very low a decennium ago, now due to the discovery of radioactive elements and their transformations it is in close proximity, and in fact the observation of the strong persistence of heat production by radio-active substances gives almost a direct support for the assumption, that the source of that heat is just nothing else than the latent energy of the atoms. In accordance to (48), a large latent energy is connected with a large mass as well. This is well in line with the fact that the radio-active elements have a particularly high atomic weight, and that their bindings belong to those with the highest specific weights.

According to J. 1 gr of radium, when surrounded by a sufficiently thick layer of lead, gives 134.4 · 225 gr = 30240 gr cal per hour. This gives according to (48) a reduction of mass in an hour by

or in a year a reduction of mass around 0.012 mgr. This amount, however, particularly with regard to the high atomic weight of radium, is still so small that it is at first out of the realm of possible experience.

Incidentally, it might appear doubtful from the outset whether a weighing scale is the right instrument for this measurement. Because the relation (48) does not apply to ponderable, but to the inertial mass, and it has already been stressed in the introduction that these two factors are by no means identical, at least not if we attribute no gravitational effect to black cavity radiation in a vacuum, although it surely has inertia. However, inertia and gravitation are in every respect and by all our experience, for the most varied materials and from the lightest to the heaviest, so closely connected with each other that we may seek without concern the origin of these two effects at the same place, namely in the latent energy of the chemical atoms. Assuming that gravity is directly proportional to the latent energy, then the mass depending on temperature