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netary body must possess its respective share, provided \ve rest our 

 reasoning upon the known properties of matter. The author, there- 

 fore, thought it deserving of consideration, whether, in any instance, 

 any deficiency of atmosphere could be proved, and whether, from 

 such source, any conclusive argument could be drawn in favour of 

 ultimate atoms of matter in general ; for since the chemical law of 

 definite proportions applies to all forms of matter, if it can be proved 

 that any one body consists of particles no longer divisible, we then 

 can scarcely doubt that all others are similarly constituted. In re- 

 spect to the non-existence of an atmosphere round the moon, Dr. 

 Wollaston observes, that the quantity of such matter retainable by a 

 body of the moon's magnitude, could not give rise to any phenomena 

 observable by our instruments ; that we should therefore look for in- 

 formation in an opposite direction, and examine that body which has 

 the greatest power ; if we there find no appearance of an atmosphere, 

 we may infer that our own is peculiar to the earth. In respect to 

 the sun, for instance, if we calculate at what apparent distance from 

 his body his force is equal to that of gravity at the surface of the 

 earth, it is there that his power would be sufficient to accumulate, 

 from an infinitely divisible medium filling all space, an atmosphere 

 fully equal in density to our own, and therefore produciag a refrac- 

 tion of more than 1 in the passage of rays obliquely through it. 



Dr. Wollaston then proceeds to show, from a detail of observations 

 of the passage of Venus near the sun in superior conjunction, which 

 took place in May last, that no such retardation in the motion of 

 that planet could be perceived in her progress toward the sun, as 

 would occur from increasing refraction, and that the phenomenon 

 does not offer the least evidence of the existence of a solar atmo- 

 sphere. 



After some suggestions respecting the best means of pursuing this 

 investigation, Dr. Wollaston observes, that he has dwelt perhaps 

 more upon the consideration of a solar atmosphere than may seem 

 necessary to those who will consider the phenomena of the occulta- 

 tions of Jupiter's eatellites by the body of the planet, the approach 

 of which is regular, till they appear in contact, instead of being re- 

 tarded by the refraction arising from an atmosphere so extensive as 

 Jupiter should attract to himself, from an infinitely divisible medium 

 filling space. 



Since the mass of Jupiter is 309 times that of the earth, the distance 

 at which his attraction is equal to gravity must be about 17'6 times 

 the earth's radius ; and since his diameter is nearly 1 1 times greater 

 than that of the earth, I '6 time his own radius will be the distance 

 from his centre at which an atmosphere equal to our own should oc- 

 casion a refraction exceeding 1. To the fourth satellite this distance 

 would subtend an angle of about 3 37'; so that an increase of den- 

 sity to 3 - 5 times that of our common atmosphere would be more than 

 sufficient to render the fourth satellite visible when behind the cen- 

 tre of the planet, and consequently to make it appear on all sides at 



VOL. II. M 



