1 84 LIGHT SCIENCE FOR LEISURE HOURS. 



small planets as Mars or Mercury, though begun much 

 earlier, had gathered in their entire substance. It 

 seems indeed not at all improbable that neither Jupiter 

 nor Saturn have quite passed through even the first 

 stage of planetary development, the ring-system of 

 Saturn being suggestive of matter as yet not completely 

 worked-up, so to speak, in that planet's system. But 

 whatever uncertainty rests on this question there is 

 none as to the original intense heat of those larger 

 planets. They must have been far hotter when first 

 formed than was our earth at the corresponding stage 

 of her development. Nor is it at all open to doubt 

 that each stage of cooling would be much longer in the 

 case of these planets than the corresponding stage of 

 our earth's cooling. 1 Jupiter contains 340 times as 

 much matter as the earth, so that if the two orbs were 

 of the same density Jupiter would have a diameter 

 seven times as great, and a surface about forty-nine 

 times as great, as the earth's. He would radiate, 

 therefore, if at the same temperature, forty-nine times 



1 The argument here used was first advanced by Sir Isaac Newton. 

 ' A globe of iron an inch in diameter,' he says, ' exposed red hot to the 

 open air, will scarcely lose all its heat in an hour's time ; but a greater 

 globe would retain its heat longer in the proportion of its diameter, 

 because the surface (in proportion to which it is cooled by the contact of 

 the ambient air) is in that proportion less in respect of the quantity of 

 the included hot matter ; and therefore a globe of red hot iron equal to 

 our earth, that is, about 40,000,000 feet in diameter, would scarcely cool 

 in an equal number of days, or in about 50,000 years. But I suspect 

 that the duration of heat may, on account of some latent causes, in- 

 crease in a yet less proportion than that of the diameter ; and I should 

 be glad that the true proportions were investigated by experiments.' 

 Buffon (according to Bailly)made experiments of the kind, with results 

 confirming Newton's opinion. 



