632 v~ 



tfcat In thft rontro of t!io p.irth ; :-'vl r.>t the 

 density of tho si;n, o\vir.g pro'iunty to if.,s 

 enormous tcmpuratr.ro, is 1 -isr. Ibuu a quarter 

 of the mean density of th oar;h. 



Wo may therefore assume vitlx groat prob- 

 ability thut the nun will still continue in its 

 condensation, oven if it only uttaiuod (hj 

 density of the earth though it will prob- 

 ably become far denser in the interior owin^ 

 to the enormous pressure this would develop 



Fn. 10. 



fresh quantities of heat, which would be 

 sufficient to maintai a for an additional 17,- 

 000,000 of yeara the same intensity of sun- 

 shine as that which is now the source of all 

 terrestrial life. 



The smaller bodies of our system might be- 

 come less hot than the sun, because the at- 

 traction of the fresh masses would be feebler. 

 A body like the earth might, if even we put 

 its thermal capacity as high as that of water, 

 become heated to even 9000 degrees, to more 

 than our names can produce. The smaller 

 bodies must cool more rapidly as long as 

 they are still liquid. The increase in tem- 

 perature, with the depth, is shown in bore- 

 holes and in mines. The existence of hot 

 wells and of volcanic eruptions shows that 

 ill the interior of the earth there is a very 

 high temperature, which can scarcely be any- 

 thing than a remnant of the high temperature 

 which prevailed at the time of its production. 

 At any rate, the attempts to discover for the 

 internal heat of the earth a more recent 

 origin in chemical processes, have hitherto 

 rostix] on very arbitrary assumptions ; and, 

 iiwiii;arc'd with the general uniform distribu- 

 tion of the internal hefct, are somewhat iri- 

 uutucieiit. 



On the other hand, considering the huge 

 masses of Jupiter, of Saturn, of Uranus, and 

 of Neptune, their small density, as well as 

 that of the eun, is surprising, while the 

 (smaller planets and the moon approximate 

 to the density of the earth. We are here re- 

 minded of the higher initial temperature, and 

 the slower cooling, which characterizes large 

 masses.* Tho moon, on tho contrary, ex- 

 hibits formations on its surface which are 



* Mr. Zix-i'ix-r concl di- from photometric meaa- 

 nrenientB, which, however, iiewl confirmation, that 

 Jupiter tlill possesses a Julit of its owu. 



~~"'*~**O i T"V "r* - f>T7C 



. .A/ J*< JLi v* 1 L IvISS. 



strikingly suggestive of volcanic craters, and 

 t'.unt to a former state of ignition of our sut- 

 Iili.0. The mode of its rotation, moreover, 

 that it, always turns tho same side to ward the 

 i:;t:-th, i.-i a peculiarity which might Inive been 

 produced by the friction of a fluid. At pres- 

 ent no truce of such a one can be perceived. 



You see, thus, by what various paths we are 

 constantly led to the same primitive con- 

 ditions. The hypothesis of Kant and La- 

 place is seen to bo one of the happiest ideas 

 in science, which at first astounds us, and 

 then connects us in all directions with other 

 discoveries, by which the conclusions are 

 confirmed until we have confidence in them. 

 In this case another circumstance has con- 

 tributedthat is, the observation that this 

 process of transformation, which the theory 

 in question presupposes, goes on still, though 

 on a smaller scale, seeing that all stages of 

 that process can still be found to exist. 



For, as we have already seen, the larger 

 bodies which are already formed go on in- 

 creasing with the development of heat, by the 

 attraction of the meteoric masses already 

 diffused in space. Even now the smaller 

 bodies are slowly drawn toward the sun by 

 the resistance in space. We still find in the 

 firmament of fixed stars, according to Sir J. 

 Herschel's newest catalogue, over 5000 nebu- 

 lous spots, of which those whose light is 

 sufficiently strong give for the most part a 

 colored spectrum of fine bright lines, as they 

 appear in the spectra of the ignited gases. 

 The nebulas are partly rounded structures, 

 which are called planetary nelmlm 'Fig. 101 ; 

 sometimes wholly irregular in form, as the 

 large nebula in Orion, represented in Fig. 11 ; 

 they are partly annular, as in the figur. , in 

 Fig. 12, from the Canes Venatici. They arc 

 fo-r the most part feebly luminous over their 

 whole surface, while the fixed stars only ap- 

 pear as luminous points. 



In many nebulas small stars can be seen, 

 as in Jb'i^s. 13 and 14, from Sagittarius and 

 Aurigo. More stars are continually being 

 discovered in them, the better are tho tele- 

 scopes used in their analysis. Thus, before 

 the discovery of spectrum analysis, Sir W. 

 Herschel's former view might be regarded s 

 the most probable, that that which we see i,a 

 be nebula) are only heaps of very fine stars, 

 of other Milky Ways. Now, however, sp&o 

 truni analysis has shown a gas spectrum in 

 many nebulas which contains stars, while* 

 actual heaps of stars show the continuous 

 spectrum of ignited solid bodies. Nebulas 

 have in general three distinctly recognizable 

 lines, one of which, in the blue, belongs to 

 hydrogen, a second in bluish-green to nitro- 

 gen,* while the third, between the two, is of 

 unknown origin. Fig. 15 shows such a 

 spectrum of a small but bright nebula in the 

 Dragon. Traces of other bright lines are 

 seen along with them, and sometimes also, as 

 in Fig. 15, traces of a continuous spectrum ; 

 all of which, however, are too feeble to admit 

 of accurate investigation. It must be ob- 

 served here that the light of very feeble ob- 

 jects which give a continuous spectrum aro 



