Life Conditions in other Worlds. 89 



prodigious attractive power, notwithstanding its mean density- 

 is, as compared with that of the earth, as 243 to 1000. A 

 mass weighing 100 lbs. on the earth would weigh from 276 to 

 224 lbs. on Jupiter, according to whether it was placed, at the 

 poles or the equator. The surface of Jupiter may be of very 

 small density, as the mean density is low, and " there is reason 

 to think," as Mr. Breen says, " that it is denser at the centre 

 than at the surface." Jupiter receives about one twenty-seventh 

 of our light and heat. If animated beings live upon its surface, 

 they must probably be of very light construction, as we may 

 suppose dense bodies would sink into it, and perhaps tumble 

 down, by mere force of gravity, through successive strata of 

 this remarkable globe. 



Saturn has an average density of about three quarters that 

 of water ; and if we imagine that it is not homogeneous, but 

 stratified, its external layers must be composed of very delicate 

 materials. Sulphuric ether has a specific gravity of *724, 

 water being 1 ; and this fact will assist comparison. Saturn 

 receives ninety-one times less light and heat than we do, but, 

 from its high reflective power, looks brighter .than would have 

 been supposed. 



If we assume that gravity and muscular activity stand 

 related to each other in the various planets as they do upon 

 earth, we shall find, as Sir John Herschel observes, " that the 

 efficacy of gravity in counteracting muscular power and re- 

 pressing animal activity on Jupiter, is nearly two and a-half 

 times that on the earth; on Mars not more than one-half; on 

 the moon one-sixth ; on the smaller planets probably not more 

 than one-twentieth, giving a scale of which the extremes are 

 in the proportion of sixty to one.'" 



Unless analogy breaks down altogether, the amount of 

 energy displayed by living objects in any planet will bear a 

 constant relation to the quantity of heat it employs in its life 

 •processes. We cannot say to the amount of heat it receives, 

 because planets may differ very greatly in the proportion of 

 heat which they retain for home use, out of the total quantity 

 that reaches them. We see this from what takes place on 

 the earth. A dry mountain height receives a large amount of 

 solar heat, and gets rid of it by violent radiation ; while at a 

 lower level, and with a moist atmosphere surrounding it like a 

 blanket, other portions of the globe that receive less keep 

 themselves very far above the lowest temperature of the 

 regions just named. Sir Jofin Herschel points out in his 

 Outlines of Astronomy that we cannot absolutely conclude 

 that objects on Mercury must be seven times as hot, because 

 that planet receives seven times as much solar radiation as we 

 do. Nor are we to assert positively that Neptune is nine 



