136 CORRELATION OF PHYSICAL FORCES. 



So universal is the tendency of matter to diffuse itself 

 into space , that it gave rise to the old saying that nature 

 abhors a vacuum ; an aphorism which, though cavilled at and 

 ridiculed by the self-sufficiency of some modern philosophers, 

 contains in a terse, though somewhat metaphorical, form of 

 expression a comprehensive truth, and evinces a large extent 

 of observation in those who, with few of the advantages which 

 we possess, first generalised by this sentence the facts of 

 which they had become cognisant. 



It has been argued that, if matter were capable of infinite 

 divisibility, the earth's atmosphere would have no limit, and 

 that consequently portions of it would exist at points of space 

 where the attraction of the sun and planets would be greater 

 than that of the earth, and whence it would fly off to those 

 bodies and form atmospheres around them. This was sup- 

 posed to be negatived by the argument of the well-known 

 paper of Dr. Wollaston ; in which, from the absence of nota- 

 ble refraction near the margin of the sun and of the planet 

 Jupiter, he considered himself entitled to conclude that the 

 expansion of the earth's atmosphere had a definite limit, and 

 was balanced at a certain point by gravitation : this deduc- 

 tion has been shown to be inconclusive by Dr. Whewell, and 

 has also been impugned upon others grounds by Dr. Wilson. 

 There is a point not adverted to in these papers, and which 

 Wollaston does not seem to have considered, viz. that there 

 is no evidence that the apparent discs of the sun and of Jupi- 

 ter show us their real discs or bodies. Sir. W. Herschel 

 regards the margin of the visible discs as that of clouds or a 

 peculiar state of atmosphere, and the rapidly changing char- 

 acter of the apparent surfaces render some such conclusion 

 necessary. If this be so, refraction of an occulted star could* 

 not be detected at all events, in the denser portion of the 

 atmosphere. 



Sir W. Herschel's observations go to prove that the 

 eun and Jupiter have dense atmospheres, while Wollaston's 



