aSz CcmttipiiH of Pkojpim-iii in ^aciis. 



line, which indlc:ttes the utmoft degree of rarcfadion of the air which I have ever obfcrved 

 ■to take place, the rarefied air ftill poflefles i 300th of the dcnfity of the air of the atmo- 

 fphere, in cafe the mercury in the barometer be fuppofcd at 30 inches elevation. 



It is certainly very fingular, that the fmall ijuantiiy of oxygenc gas which remains in air 

 fo rarefied fliould he fufficient for the inflammation of phofphonis ; more tfpecially as all 

 other combuflible fubftances are cxtinguiflied in air rarefied to a much lower degree. I 

 have already explained the principal reafon. There is another circumftancc which probably 

 favours this inflammation, of which I fhall fpeak hereafter, when I (hall have made fome 

 experiments. 



X The phenomenon exhibited by the phofphorus in rarefied air, as here defcribed, is 

 certainly a real combufiion, as is proved by the very remarkable diminution of weight of tlie 

 phofphorus after the experiment has been made in a large receiver, or has been feveral 

 times repeated. The phofphoric acid produced by the combination of oxygenc with the 

 fubflance burned is alfo found on the plate of the air pump. 



XI. The combuftion of phofphorus in air highly rarefied is accompanied with fcvcral very 

 fingular phenomena, which I fhall here mention. 



1. The phofphorus ufually emits, a (hort time after the commencement of the inflamma- 

 tion, fiery jets, in the form of fmall ignited balls, which are difperfed on aH fides in the 

 receiver, and exhibit a very curious and furprifing appearance. I have not hitherto been 

 able to explain this phenomenon. 



2. The flame which furrounds the phofphorus when it burns in the rarefied air, extends 

 farther and farther in a globular form. Its light at the fame time becomes paler and paler, 

 and at length difappears. This enlargement of the flame, and diminution of the light, are 

 probably to be attributed to the oxygene, to which the phofphoric exhalations may unite, 

 being gradually exhaufted. Tlie light at la(l entirely difappears, when all the o.\ygene gas 

 which exifted in the rarefied air, and could be reached by the phofphoric exhalations, is 

 combined with them ; for, as foon as there is no more oxygenc to which the phofphoric 

 exhalations may unite, there ceafes to be any further feparation of caloric, which caufes 

 the flame and the illumination. 



3. When a fmall quantity of atmofpheric air or oxygene gas is fuffered to enter the 

 upper part of the receiver by a cock after the combuftion or light of the phofphorus has 

 difappeared, a pale light is then feen to difperfe itfelf through the whole capacity of the re- 

 ceiver This light mud certainly be attributed to the combination of the oxygene, of the 

 gas which has entered, with the phofphoric exhalations which exift in the receiver. The 

 exhalations which the phofphorus emits after it has been heated by the inflammation, are 

 apparently more fubtle and light than thofe which preceded that phenomenon ; and it is 

 probably owing to this reafon that they can fupport themfelves in the gas which remained in 

 the receiver, though very highly rarefied. 



4. When the apparatus is left untouched for fome time after the combuRion of the phof- 

 phorus in the rarefied air has ceafed, and the whole has become cooled, then the phofphoric 

 exhalations defcend in this rarefied air. In proof of this, when the experiment is made in 

 a receiver, on the plate of the air pump, and air is fuflxred to enter by a cock beneath the 

 plate, as ufual, it is then feen that the light occafioned by the entrance of the air takes place 

 only near the plate upon which the phofphoric exhalations hare fallen. 



XII. 



