Page:Popular Science Monthly Volume 9.djvu/699

Rh from an infinite distance would produce 6,000 times more heat from concussion than it would generate by its combustion. An idea of the amount of energy exerted by one pound weight falling into the sun will be conveyed by stating that it would be sufficient to hurl the Warrior, with all its stores, guns, and ammunition, over the top of Ben Nevis! But, if we accept gravitation as the source of energy, we accept a cause, the value of which can be mathematically determined with very considerable accuracy.

The amount of heat given off by radiation in a year is known; the total amount of work performed by gravitation in condensing a nebulous mass to an orb of the sun's present size is known. The result is, that the amount of heat thus produced by gravitation would suffice for about twenty millions and a quarter of years. This is on the assumption that the nebulous matter composing the sun was originally cold, and that heat was generated in it by the process of condensation only. It is, however, quite conceivable that the nebulous mass possessed a store of heat previous to condensation, and that the very reason why it existed in the gaseous condition was that its temperature was excessive. The particles composing it would have had a tendency, in virtue of gravitation, to approach one another if they had not been kept apart by the repulsive energy of heat; it is not, then, unreasonable to suppose that the attenuated and rarefied mass was vaporous by reason of heat, and began to condense only when its particles began to cool. By the known laws under which heated gases condense, the amount of heat originally possessed by the gas bears a definite and known proportion to the amount of heat generated by condensation; and, on the assumption that the analogy holds good in the case of the sun, which holds in the condensation of other heated gases, nearly fifty millions of years' heat must have been stored up in the mass as original temperature. This, added to the twenty and a quarter millions which resulted from gravitation, gives rather more than seventy millions of years' sun-heat.

As, however, this quantity gives the total amount of heat given out by the mass since it began to condense, the earth could not have had an independent existence till long after that time. The sun must have had time to condense from its outer limits as a nebula, to within