i 



H, Hehnholtz on the Interaction of Natural Forces. 215 



the time during which the first nebulous mass condensed into 



our planetary system, our most daring conjectures must cease. 



I The history of man, therefore, is but a short ripple in the ocean 



of time. For a much longer series of years than that during 

 "which man has already occupied this world, the existence of the 

 present state of inorganic nature favorable to the duration of 

 man seems to be secured, so that for ourselves and for long gen- 

 erations after us we have nothing to fear. But the same forces 

 of air and water,' and of the volcanic interior, which produced 

 former geologrical revolutions, and buried one series of living 

 forms after another, act still upon the earth's crust. They more 

 probably will bring about the last day of the human race than 

 those distant cosmical alterations of which we have spoken, and 

 ^perhaps force us to make way for new and more complete living 

 forms, as the lizards and the mammoth have given place to us 

 and our fellow-creatures which now exist. 



Thus the thread which was spun in darkness by those who 

 sought a perpetual motion has conducted us to a universal law 

 of nature, which radiates light into the distant nights of the be- 

 ginning and of the end of the history of the universe. To our 

 Own race it permits a long but not an endless existence; it 

 threatens it with a day of judgment, the dawn of which is still 

 happily obscured. As each of us singly must endure the thought 

 of his death, the race must endure the same. But above tlie 

 forms of life gone by, the human race has higher moral problems 

 before it, the bearer of which it is, and in the completion of 

 which it fulfills its destiny. 



iVo^e to page 205.— I must here explain the calculation of the heat 

 which must be produced by the assumed condensation of the bodies of 

 our system from scattered nebulous matter. The other calculations, the 

 results of whicli I have mentioned, are to be found partlyjn J. R. Mayer's 

 papers, partly in Joule's conimunications, and partly by aid of the known 

 facts and method of science : they are easily performed. 



The measure of the work performed by the condensation of tlie mass 

 from a state of infinitely small density, is the potential of the condensed 

 mass upon itself. For a sphere of uniform density of the mass M, and the 

 yadius R, the potential upon itself V— if we call the mass of the earth ??z, 

 its radius r, and the intensity of gravity at its surface ^,—has the value 



,^ 3 r2M2 



5 R//1 

 Let us regard the bodies of our system as such spheres, then the total 

 • "Work of condensation is equal to the sum of all their potentials on them- 

 ^Ives. Asj however, these potentials for different spheres are to each 



^ther as the quantity -jp, they all vanish in comparison with the sun ; 



even that of the greatest planet, Jupiter, is only about the one hundred- 

 thousandth part of that of the sun; in the calculation, therefore, it is 

 only necessary to introduce the latter. 



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