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POPULAR SCIENCE REVIEW. 
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.* 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 as much heat ; but he would 
have about 340 times as much heat to part with for each degree 
of cooling ; hence his rate of cooling would be slower in 
the proportion of about 7 to 1. Jupiter appears actually to- 
have a much greater volume than has been here supposed, his 
diameter exceeding that of the earth nearly eleven times, and 
his surface exceeding hers about 115 times. This would still 
leave his rate of cooling slower in the proportion of about three 
to one. But inasmuch as it is certain that if formed of the 
same material, Jupiter, when at the same stage of cooling, 
would be much denser than the earth (because of his greater 
attractive energy), our assumption rather falls short of the 
truth than exceeds it. The argument next to be considered 
will sufficiently indicate this. To complete the present argu- 
ment it is only necessary to add that the various stages of 
cooling through which our earth has already passed have cer- 
tainly required hundreds of millions of years, wherefore the 
corresponding stages for Jupiter would require seven times as 
many hundreds, and the total period required by Jupiter to* 
* The argument here used was first advanced hv 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, increase 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. 
