NEWS FROM JUPITER. 437 



the aspect of Jupiter with adequate telescopic power. We have in 

 Jupiter's mean density an argument of irresistible force against the 

 only view which enables us even hypothetically to escape from the 

 conclusions just indicated. Let it be granted, for the sake of argu- 

 ment, that Jupiter's cloud-layer is less than fourteen miles in depth, so 

 that we are freed for the moment from the inference that at the lower 

 part of the atmosphere there is either an intense heat or else a density 

 and pressure incompatible with the gaseous condition. We cannot, in 

 this case, strike off more than twenty-eight miles from the planet's 

 apparent diameter to obtain the real diameter of his solid globe — 

 solid, at least, if we are to maintain the theory of his resemblance to 

 our earth. This leaves his real diameter appreciably the same as his 

 apparent diameter, and as a result we have the mean density of his 

 solid globe equal to a fourth of the earth's mean density, precisely as 

 when we leave his atmosphere out of the question. Now, I apprehend 

 that the time has long since passed when we can seriously proceed at 

 this stage to say, as it was the fashion to say in text-books of astron- 

 omy, " Therefore the substance of which Jupiter is composed must be 

 of less specific gravity than oak and other heavy woods." We know 

 that Brewster gravely reasoned that the solid materials of Jupiter 

 might be of the nature of pumice-stone, so that, with oceans resem- 

 bling ours, a certain latitude was allowed for increase of density in 

 Jupiter's interior. But, in the presence of the teachings of spectro- 

 scopic analysis, few would now care to maintain, as probable, so pre- 

 posterous a theory as this. Every thing that has hitherto been learned, 

 respecting the constitution of the heavenly bodies, renders it quite 

 unlikely that the elementary constitution of Jupiter differs from that 

 of our earth. Again, it was formerly customary to speak of the pos- 

 sibility that Jupiter and Saturn might be hollow globes, mere shells, 

 composed of materials as heavy as terrestrial elements. But, what- 

 ever opinion we may form as to the possibility that a great intensity 

 of heat may vaporize a portion of Jupiter's interior, we know quite 

 certainly that there must be enormous pressure throughout the whole 

 of the planet's globe, and that even a vaporous nucleus would be of 

 great density. For it is to be remembered that all that I have said 

 above respecting the possibility of gases existing at great pressures 

 applies only to ordinary temperatures — such temperatures, for example, 

 as living creatures can endure. At exceedingly high temperatures 

 much greater pressure, and therefore much greater density, can be at- 

 tained without liquefaction or solidification. And, in considering the 

 effect of pressure on the materials of a solid globe, we must not fall 

 into the mistake of supposing that the strength of such solid materials 

 can protect the material from compression and its effects. We must 

 extend our conceptions beyond what is familiar to us. We know that 

 any ordinary mass of some strong, heavy solid — as iron, copper, or 

 gold — is not affected by its own weight so as to change in structure to 



