MITHOD OF CLEARIKG OBSERVED LUNAR DISTANCES, &C. 43 



This fubftance diftilled in a retort, did not now emit any The refidue of 

 ammoniacal odour, nor did it produce any fluid. The refidue this when dif- 



,.„.,, . r i-ii i- tilled was hard 



oi the dittillation was of a grey colour, which became white like porcelain 



by calcination, and by the intenfe heat of the fire it acquired a " d «femble4 



the hardnefs of porcelain ; it no longer had the alcaline tafte, ename |, & ? , 



it no longer decompofed muriate of ammonia ; it no longer 



changed fyrup of violets green, water no longer a6led on it ; 



it lhort, the enamel not diftilled and fubmitted to thefe latter 



experiments, afforded the fame refults as the refidue of the 



diftillation : I again placed fome in digefHon with diluted ful- 



phuric acid, for the purpofe of treating it like the offeous fub- 



ftances from which phofphoric acid is to be feparated, and I It feemed to be 



obtained umilar refults ; that is to fay, acid phofphate of ii me phofphatoflirae, 



in folution in the liquor, with fulphate of lime, and fragments 



of the enamel not decompofed. Thefe are the falts whofe 



prefence has been afcertained from examinations by the ufual 



methods. 



From the great difference of thefe refults obtained in both The enamel 

 methods of treating the enamel of the* teeth, it will be feen J e P a f ated .^. 



° iulpn. acid it 



that at firft I was led into an error by the effect of the ful- not pure, 

 phuric acid, which, in the preliminary preparation left con- 

 siderable portions of enamel, whofe physical characters caufed 

 me to believe them unchanged. 



I have thought it ufeful for the interefr. of fcience, to make 

 known thefe furprifing differences, they may be advantageouily 

 applied in the medical art, and aflift. in explaining the pheno- 

 mena, as I had occafion to point out elfe where. 



IX. 



A New Method of Clearing obferved Lunar Dijlances of the Ef- 

 fects of Parallax and Refraction, for the Purpofe of determining 

 the Longitude at Sea or Land. By Mr. J. Andrew. Com* 

 municatcd by the Author. 



JLrfET A, B, C, D, PI. 3. Fig. 3. be four given points on Principle of the 

 any fphere, and fo conflicted that they mall all lie on the fame meth ° d de ™>»- 

 imaginary plane; and let the fphere be fuppofed to be cut fo 

 by this plane, that the lines AB, BC, CD, DA, and BD, 

 joining the given points, may be all of them right lines, or 

 chords of great circles which may be fuppofed to have paffed 



through 



