Another consequence of these discoveries was that any variation in the internal energy of a body maintained at rest, such as would result from compressing it, heating it or electrifying it, would be accompanied by an increase in its mass at rest.[50] This discovery enabled us to account for a curious discrepancy in the comparative masses of [Pg 158] the helium and of the hydrogen atom, a mystery which had never been accounted for by classical science. To illustrate: The nucleus of the hydrogen atom is composed of one proton; and the nucleus of the helium atom is composed of four protons, held together by two electrons. The protons alone having a mass of any importance, it seemed logical to assume that the mass of the helium atom would be four times that of the hydrogen atom. Accurate measurements proved, however, that there was a slight deficiency, and that the mass of the helium atom was less than four times that of the hydrogen atom. Classical science had recognised that the closer the electrons and protons were packed together in the nucleus, the smaller would be the electric energy of the atom, but it appeared impossible to account for the deficiency in mass. The theory of relativity by its identification of mass and energy gave an obvious solution to the problem. The deficiency in mass of the helium atom was to be ascribed to the loss of energy radiated under the form of heat and light when the electrons and protons were being packed together. At the same time it became possible to calculate with precision from the loss of mass the precise amount of energy that had been liberated. It followed that in regions of the cosmos where the helium atoms were in process of formation owing to the grouping together of the protons and electrons, enormous amounts of heat and light would be radiated continually. This consequence of the theory enabled astrophysicists to account for the enormous temperature of the sun, maintained through thousands of millions of years in spite of incessant radiation in a frigid space. Helium atoms are in constant process of formation in the sun, and in this fact resides the source of its apparently inexhaustible supply of energy. [Pg 159] Through this constant radiation of energy, the sun and stars would be gradually losing in mass; for light was of course a form of energy. As such, light should possess mass and momentum and exert a pressure over bodies on which it impinged. This pressure, indeed, was not unknown, since it resulted from Maxwell’s equations and had been verified experimentally by Lebedew and by Nichols and Hull. As a matter of fact it was to this pressure of light that the apparent repulsion of comets’ tails away from the sun was supposed to be due. Nevertheless, though not a new discovery, the fact that Einstein’s theory entailed momentum for light constituted one of the many indirect confirmations of the theory.