REMARKS ON COMBUSTION. 99 



Acid is poured upon lime*. In both cafes a femi-combuftion Semi-combuf- 

 takes place. The water and the acid being folidified give JJ™ ° f hme and 

 out caloric, while the quicklime gives out light; that lime 

 during its calcination combines with light, and that light is a 

 component part of quicklime is demonftrated by the following 

 experiment, for which we are indebted to Scheele. 



It is well known that fluor fpar (native fluate of lime) has Phofphorefcence 

 the property of phofphorefcing ftrongly when heated, bat that p lain £j r par e ** 

 the experiment does not fucceed twice with the fame fpeci- 

 men. After it has been once heated fufficiently, no fubfe- 

 quent heat will caufe it to phofphorate. Now phofphoref- 

 cence is merely the emiffion of light, light of courfe is a com- 

 ponent part of fluor fpar, and heat has the property of fepa- 

 rating it. But the phofphorefcing quality of the fpar may be 

 again recovered to it, or which is the fame thing, the light 

 which the fpar had loll may be reftored by the following pro- 

 cefs. Decompofe the fluate of lime by fulphuric acid, and reft ored» 

 preferve the fluoric acid feparated. Boil the fulphate of lime 

 thus formed with a fufficient quantity of carbonate of foda; a 

 double decompofition takes place ; fulphate of foda remains in 

 folution, and carbonate of lime precipitates. Calcine this 

 precipitate in a crucible till it is reduced to quicklime, and 

 combine it with the fluoric acid to which it was formerly 

 united. The fluor fpar thus regenerated phofphorefces as at 

 firfl. N Hence the lime during its calcination mnft have com* 

 bined with light. 



That potafti contains light, may be proved in the fame man- Light in potato.' 

 ner as the exiftence of that body in quicklime. Dize has 

 fiiown that much light is emitted when fulphuric acid is poured 

 upon potafh, but more when it is poured on the carbonate of 

 potaih. Now as potafti is deprived of its carbonic acid by 

 lime, it is obvious that the procefs muft be a double decom-* 

 pofition ; the bafe of the lime combines with carbonic acid, 

 while its light combines with the potafh. 



Thefe remarks on femi-combuftion might eafily be extended 

 much farther. For it is obvious, that whenever a liquid com- 

 bines with a folid containing light, and the product is a folid 

 body, fomething analogous to femi-combuftion muft take 

 place. Hence the reafon why water increafes the violence of 

 combuftion when thrown fparingly into a common lire. 

 * Dize Jour, de Phyf. 49, 177. 



H 2 XX. A 



