Note 43 — Chapter VII

Sir Isaac Newton, in his Principia, under prop. 16, book 3, lays down the following proposition, viz: that "the heat of the sun is as the density of his rays, that is reciprocally as the squares of the distances from the sun." From this principle, it has been assumed by some of our modern astronomers, that but few of the planets can be inhabited, as if the effect of light and heat are reciprocally proportionate to the squares of the distances from the centre of their propagation; and if you divide the square of the earth's distance from the sun, the quotient will show, that the light and heat, which Mercury receives, are about seven times greater, making it more than twice as hot as boiling water. The light and heat communicated to Saturn, being only the one hundredth part of that of the earth, the difference is more than seven times as great as that between our summer heat and red hot iron, if the light and heat of the sun are only in proportion to the density of his rays. Such extremes of heat and cold, we would naturally conclude must totally preclude all material being, if in the least degree resembling those we are acquainted with; nor could any of the vegetable world, known to us, germinate in either extreme; nay, even the matter of our globe would scarcely withstand it, our oceans would be dissipated in vapour, on Mercury, and frozen to the bottom on Saturn. Considerations like these must induce us to conclude, that light and heat cannot be communicated exactly on the plan laid down by Newton, viz: that the heat of the sun is simply as the density of his rays: for though the sun's rays may be the sine qua non, without which no light or heat would be communicated, yet the quantum of heat may depend on the density and co-operation of the medium through which it passes, or upon some other circumstance not known to us, and perhaps impossible for us to know.