As for the temperature of the sun, we only know that it is hot enough to turn the metals into gases in the manner in which this is done in a strong electric arc, but no satisfactory method of reckoning the scale of this heat has been devised. The probabilities are to the effect that the heat is to be counted by the tens of thousands of degrees Fahrenheit, and it may amount to hundreds of thousands; it has, indeed, been reckoned as high as a million degrees. This vast discharge is not due to any kind of burning action--i.e., to the combustion of substances, as in a fire. It must be produced by the gradual falling in of the materials, due to the gravitation of the mass toward its centre, each particle converting its energy of position into heat, as does the meteorite when it comes into the air. It is well to close this very imperfect account of the learning which relates to the sun with a brief tabular statement showing the relative masses of the several bodies of the solar system. It should be understood that by mass is meant not the bulk of the object, but the actual amount of matter in it as determined by the gravitative attraction which it exercises on other celestial bodies. In this test the sun is taken as the measure, and its mass is for convenience reckoned at 1,000,000,000. TABLE OF RELATIVE MASSES OF SUN AND PLANETS.[2] +------------------------------------------------------------+ | The sun 1,000,000,000 | | Mercury 200 | | Venus 2,353 | | Earth 3,060 | | Mars 339 | | Asteroids ? | | Saturn 285,580 | | Jupiter 954,305 | | Uranus 44,250 | | Neptune 51,600 | | Combined mass of the four inner planets 5,952 | | Combined mass of all the planets 1,341,687 | +------------------------------------------------------------+ [Footnote 2: See Newcomb's Popular Astronomy, p. 234. Harper Brothers, New York.] It thus appears that the mass of all the planets is about one seven hundredth that of the sun.