Pouillet, the French physicist, who undertook this critical investigation, arrived at certain results, which he states as follows:
"If the total quantity of heat emitted by the sun was exclusively employed to melt a layer of ice applied to the solar globe, and covering it completely in all its parts, that quantity of heat would be able to melt, in one minute, a layer of eleven metres, eighty centimetres, and in one day a layer of seventeen kilometres in thickness."
"'This same quantity of heat,' says Professor Tyndall, 'would boil 2900 milliards of cubical kilometres of water, at the temperature of ice.'"
The astronomer Herschel found, that, in order to extinguish the sun, to prevent his "giving out caloric," according to the scientific phrase, it would be necessary to dash a stream of iced water, or a cylindrical column of ice, eighteen leagues in diameter, against its surface, at a rate of speed of 70,000 leagues per second. A comparison adopted by Professor Tyndall gives us an amazing view of the intensity of the calorific force of the sun. "Imagine," says he, "that the sun is surrounded by a layer of peat, seven leagues in thickness, the heat produced by its combustion would be the same as that produced by the sun in one year." The physicists have measured the intensity of the sun's light with exactitude, as they had previously measured his heat.
It is known that the solar light is 300,000 times stronger than that of the full moon, and 765,000,000 stronger than that of Sirius, the most brilliant of the stars.
Bouguer discovered, by experiments made in 1725, that the sun, at a height of 31° above the horizon, gives a light equal to that of 11,664 candles, placed within 43 centimetres of the object to be lighted, and equal to 62,177 candles placed within one metre.
According to this result, if we take account of atmospheric absorption, and of the law of the variation of the intensity of light, which decreases in inverse ratio to the square of distance, the light given by the sun at its zenith would be 75,200 times greater than that of a single candle, placed within one metre. Wollaston had arrived at a similar conclusion. By means of experiments of another kind, made during the months of May and June, 1799, Wollaston found that 59,882 candles, at one metre, give as much light as the sun. Supposing the sun to be in the zenith, the lightening power of that great star would be equivalent to 68,009 candles.
There is but little difference between this valuation and that of Bouguer, who states the result at 75,200 candles.
Whatever may be the intensity of the light of the sun, we now possess other sources of light which approach to it. Such is the oxhydric light, produced by burning hydrogen gas by means of a current of oxygen gas, or air, a method of lighting which has recently been employed in Paris and in London. This light is equal in power to more than 200 candles. A thread of magnesium burning in the air, develops a prodigious quantity of light, which may be taken as equivalent to that of 500 candles. The electric light produced by a voltaic battery of from 60 to 80 coils, produces a luminous arc equal to the light of 800 or 1000 candles. In the latter instance the voltaic arc, according to Bouguer and Wollaston, would give 75 times less light than the sun, supposing the luminous electric point to be placed at a distance of one metre.
With very powerful batteries, it has been possible to go further, and produce a light not much inferior to that of the sun. Messieurs Fizeau and Foucault, by comparing the light of a voltaic arc, produced by the action of three series of Bunsen's coils, of forty-six couples each, with the light of the sun in a clear sky in April, have established that the light-giving power of the sun is not more than twice and a half that of the electric light.
The preceding numbers represent the light-giving power of the sun upon our globe, taking into account atmospheric absorption. Arago, on endeavouring to determine the intrinsic light-giving power of the sun, found that the intensity of the solar light is 52,000 times greater than that of a candle placed at one metre. But, according to more recent researches for which we are indebted to Mr. Edmond Becquerel, the result obtained by Arago is greatly inferior to the truth, and the light of the central star is 180,000 times greater than that of a candle placed at one metre.