VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 553 



calx of lead; and in others, the common calcareous earth, but still seems suffi- 

 ciently different from that to constitute a new genus, as will appear from a little 

 attention to the following circumstances. , 



Terra ponderosa, Terra calcarea, 



When dissolved in water, precipitates on the ad- Dissolved in water, does not precipitate on the 



dition of the smallest portion of vitriolic acid. addition of vitriolic acid. 



Its gypsum therefore is insoluble. Its gypsum therefore is soluble. 



With the nitrous and marine acid, forms crystals With nitrous and marine acids, forms salts so 



which do not deliquesce. deliquescent that they cannot be kept in a 



crystallized form. 

 Decomposes vitriolic salts via humida. Does not decompose vitriolic salts. 



It has been called terra ponderosa,* or heavy earth, on account of the great 

 specific gravity of its gypsum ; its spar is likewise heavy enough to countenance 

 such an appellation ; but the earth itself does not appear to be a heavy substance, 

 and I imagine the great weight of its compounds with the vitriolic and aerial acids 

 is owing to the absence of water. 



XXIV. Observations on the Transit of Mercury over the Suns Disc, Nov. 12, 

 1782, made at the Royal Observatory at Paris. By J. Wm. Wallot. From 

 the French, p. 312. 



Mr. W. says he observed this transit along with M. Cassini, with a 3-feet 

 achromatic telescope by Dollond. The transit was attended by two unfavourable 

 circumstances, the sun's proximity to the horizon, and the near passage of Mer- 

 cury to the sun's limb. But the fine weather made some compensation, and en- 

 abled them to make good observations, which are reduced to the true time of 

 the meridian of Paris observatory. The times of the immersion and emersion 

 are given as below : viz. at 



2 h 56 m 28 s .8 the first exterior contact, 



28 .8 half immersed, or 5 's centre on the sun's limb, 



3 .8 total immersion, or internal contact, 

 45 .8 Mercury quite detached from the limb, 

 18 .4 interior contact at the exit, 

 36 .4 half emersed, or $'s centre on the ©'s limb, 

 53 .4 exterior contact, or end of the transit. 

 In measuring the planet's diameter when on the sun's disc, Mr. W. found it 

 twice the same quantity, viz. 9 parts of the objective micrometer, which are 

 equal to 9". 535 of a degree. The sun's limb was so undulatory that Mercury, 

 towards the final exit, exactly resembled a body floating on the waves of water 

 much agitated, frequently disappearing and appearing again. Mr. W. saw no- 



* Termed by subsequent chemists, barytes. 

 VOL. XV. 4 B 



