Chap. 6.] Accenfan of Pkojptorus. 141 



tion, will be produced. Of this nature is the matter 

 of phofphorus, and that very common or almoft uni- 

 verfal fubftance, which is diftinguiihed by the name of 

 hydrogen, or inflammable air. Thus if a quantity of 

 phofphorus is expofed to the atmofphere, it will abforb 

 a confiderable quantity of the pure part of the air, and 

 by the union be converted into phofphoric acid. In 

 the mean time the caloric will be detached from the 

 air i and, provided the air is well charged with heat, 

 ignition or accenfion will be produced on the farface 

 of the phofphorus. Inflammable air (or bodies con- 

 taining that principle) has, however, either a weaker 

 attraction for air than phofphorus has, or its particles 

 have a flronger attraction for each other. It is ne- 

 cefiary therefore that it fhould be prefented to the air 

 in a rarer ftate ; and a degree of internal agitation, and 

 even a third attractive power, are required to effect 

 the union of the two fubflances, and the detaching of 

 the fire from them. Thus, for inftance, if pure air is 

 mixed with a quantity of inflammable air, the electric 

 fpark, or a fmall quantity of fire in fome form or other, 

 muft be introduced to effect their accenfion. In this 

 cafe a double attraction takes place. The pure and 

 inflammable airs unite together, or are condenfed into 

 a fluid, and the matter of fire, which is introduced, 

 carries with it the fire which is detached from the two 

 airs, and thus a complete ignition is produced. In 

 the ordinary procefs of burning, when a quantity of 

 inflammable or combuftible fubftances are heaped to- 

 gether, and fire introduced among them, by the ac- 

 tion of the fire the inflammable part is firft expanded 

 from its folid ftate into a ftate of inflammable vapour, 

 it corhes neceflarily into contact with the pure air of 

 ihe atmoiphere, and the action of the fire ftill conti- 

 nuing; 



