CONDITION OF THE LARGER PLANETS. 
41 
chapters on the larger planets — only reasoning which depends on 
the assumptions which had been made by those whom Whewell 
proposed to controvert. In fact his theory may be regarded, 
and was probably regarded by himself, as merely a reductio ad 
absurdum of the unreasoning faith of those who had long held 
unchallenged the belief in the habitability of all the planets. 
I proceed to indicate the leading arguments for the theory 
that Jupiter and Saturn are still intensely hot, noting first that 
I do not propose to discuss the details of the various arguments* 
(which I have already done elsewhere), and secondly that the 
arguments are not dependent one upon the other, but severally 
independent, so that if any seem weaker than the rest, the con- 
clusion is not on that account invalidated, but the weight of 
evidence only pro ianto diminished. It is important to notice 
this, because many who, in examining a series of arguments, 
recognize, or suppose they recognize, some weakness in the 
evidence of one or other argument, are apt to infer that the 
conclusion is to the same degree invalidated as it would be if 
the arguments were dependent, and therefore each one essential 
to the establishment of the conclusion. 
The first argument for the theory is that derived from the 
now accepted hypothesis of the growth or development of the 
solar system. It is rendered to all intents and purposes certain, 
as well from the evidence of the earth’s crust, as from that 
given by the movements of the sun, planets, asteroids, and 
satellites, that the solar system was developed from a former 
nebulous condition. The process of development may have 
been that conceived by Laplace in his nebular hypothesis, which 
may be described as the contraction theory, or that recently 
suggested by meteoric discoveries, which may be called the ac- 
cretion theory, or, far more probably, the solar system was formed 
by combined processes of contraction and accretion. But in 
■any case the planets as severally formed were intensely heated, 
partly vaporous, partly liquid bodies, the larger being the more 
heated. It is no longer supposed, as in Laplace’s time was the 
case, that the outermost planets were fashioned first. They 
may have begun to be formed first — this, indeed, is altogether 
probable — but the vastness of their bulk suggests that they 
went on gathering in matter and contracting (forming in the 
process their systems of moons) long after such small planets as 
Mars or Mercury, though begun much earlier, had gathered in 
their entire substance. It seems indeed not at all improbable 
* I may, perhaps, be permitted to remark here, that the details of many 
among the arguments here indicated will be found fully discussed in my 
Lecture delivered at Glasgow on November 9 last, and published by 
Messrs, Collins, of that city. 
