VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 555 



time of the conjunction within l".6 of the same; and the nearest distance of the 

 centres, as well as the latitude of Mercury in the conjunction, l".3 too little. 

 As to the two observations of the centre of Mercury on the sun's limb, they give 

 the middle of the transit 8". 7 sooner than the interior contacts, and the nearest 

 distance of the centres, as also the latitude of Mercury in the conjunction, only 

 -i\ of a second too little. 



Though the result of the calculations agree sufficiently well, yet Mr. W. is 

 not satisfied to find the least distance of the centres l". 3 greater by the exterior 

 contacts than by the interior; a difference which shows an error in the duration; 

 either that the duration between the two exterior contacts is too small, or that of 

 the interior contacts too great. The cause of this difference he expects to find 

 in the effect of a supposed atmosphere about Mercury, or some such cause; and 

 in effect, by examination, he sees no reason to doubt of its being the effect of a 

 refraction or inflection of the solar rays in their passage near the surface of 

 Mercury. 



XXV. Thoughts on the Constituent Parts of Water and of Dephlogisticated 

 Air; with an Account of some Experiments on that Subject. By Mr. James 

 Watt* Engineer, p. 329. 



1 . It has been known for some time, that inflammable air contained much 

 phlogiston ; and Dr. Priestley has found, by some experiments made lately, that 

 it " is either wholly pure phlogiston, or at least that it contains no apparent mix- 

 ture of any other matter." In my opinion however, it contains a small quantity 

 of water, and much elementary heat.-f- He found, that by exposing the calces 

 of metals to the solar rays, concentrated by a lens, in a vessel containing inflam- 

 mable air only, the calces of the softer metals were reduced to their metallic 

 state; and that the inflammable air was absorbed in proportion as they became 

 phlogisticated ; and, by continually supplying the vessel with inflammable air, as 

 it was absorbed, he found, that out of 101 oz. measures, which he had put into 

 the vessel, 99 oz. measures were absorbed by the calces, and only 2 oz. measures 

 remained, which, on examination, he found to be nearly of the same quality the 

 whole quantity had been of before the experiment, and to be still capable of de- 

 flagrating in conjunction with atmospheric or with dephlogisticated air. There- 

 fore, as so great a quantity of inflammable air had been absorbed by the metallic 

 calces; the effect of reducing them to their metallic state had been produced; 



* The celebrated improver of the steam-engine ; to the augmented power and diversified applica- 

 tion of which, may, in a great measure, be attributed the present flourishing condition of some of 

 the most important arts and manufacturing concerns in this country. 



I Previous to Dr. Priestley's making these experiments, M. Kirwan had proved, by very ingenious 

 deductions from other facts, that inflammable air was, in all probability, the real phlogiston, in an 

 aerial form. These arguments were perfectly convincing to me; but it seems more proper to rest 

 that part of the present hypothesis on the direct experiment. — Orig. 



4 B 2 



