COMBUSTION IN AIR. 253 



in combining with oxygen, at a high temperature, always pro- 

 duce carbonic acid and water, which being volatile disappear, 

 forming part of the heated aerial column that rises from the 

 burning body. The constant removal of the product of oxida- 

 tion, thus effected by its volatility, greatly favours the progress 

 of combustion in such bodies, by permitting the free access of 

 air to the unconsumed combustible. The interference of air 

 in combustion is obvious from the facility with which a 

 fire is checked or extinguished when the supply of air is 

 lessened or withheld, and, on the contrary, revived and 

 animated when the supply of air is increased by blowing upon 

 it. For the oxygen of the air being consumed in combining 

 with the combustible, a constant renewal of it is necessary. 

 Hence, if a lighted taper, floated by a cork upon water, be 

 covered with a bell jar having an opening at top, such as that 

 in which the iron-wire was burned, the taper will burn for a 

 short time without change, then more and more feebly, in 

 proportion as the oxygen is exhausted, and at last will expire. 

 The air remaining in the jar is no longer suitable to support 

 combustion, and a second lighted taper introduced into it by 

 the opening at top, is immediately extinguished. 



In combustion, no loss whatever of ponderable matter occurs ; 

 nothing is annihilated. The matter formed may always be 

 collected without difficulty, and is found to have exactly the 

 weight of the oxygen and combustible together which have dis- 

 appeared. The most simple illustrations of this fact are ob- 

 tained in the combustion of those bodies, which afford a solid 

 product. Thus when 2 grains of phosphorus are kindled in a 

 measured volume of oxygen gas, they are found converted after 

 combustion into a quantity of white powder (phosphoric acid), 

 which weighs 4| grains, or the phosphorus acquires 2 grains ; at 

 the same time ^ cubic inches of oxygen disappear which weigh 

 exactly 2J grains. In the same way when iron wire is burned 

 in oxygen, the weight of solid oxide produced is found to be 

 equal to that of the wire originally employed added to that 

 of the oxygen gas which has disappeared. But the oxida- 

 tion of mercury affords a more complete illustration of what 

 occurs in combustion. Exposed to a moderate degree of heat 

 for a considerable time in a vessel of oxygen, that metal is 

 converted into red scales of oxide, which possess the additional 

 weight of a certain volume of oxygen which has disappeared. 



