83 . 



That fuel cannot consume without air is clearly 

 proved by an easy experiment Let a strong, hollow 

 cylinder of iron be fitted with a firm screw at each end. 

 Inclose in this a piece of charcoal : then screw up both 

 ends, and place it in a strong fire. Let it stay there as 

 long as you will. Open it when cool, and the charcoal 

 is no way diminished. It is plain from this, that the 

 consumption of fuel depends on the rarefaction and agi- 

 tation of its parts by fresh air. And hence we have the 

 reason of the known method, of extinguishing fires by 

 smothering them. 



3. The watry part of the fuel being rarefied by the 

 heat, ascends in the form of smoke, carrying with it 

 many of the lighter particles, which adhere as soot to 

 the chimney. The grosser and more compact, the con- 

 texture whereof the fire cannot wholly destroy, remain 

 arid constitute ashes, which are of consequence ex- 

 tremely porous, all that was combustible in it being con- 

 sumed. 



To enlarge a little on this subject. Fire is a body, 

 and a body in motion. It is in motion ; for it expands 

 the air, which can no otherwise be done, than by com- 

 municating motion to it. And that it is a body appears 

 hence. Pure mercury inclosed in a phial, and kept in a 

 gentle heat for a year, is reduced into a solid. And its 

 weight is considerably increased, which can only spring 

 from the accession of fire. 



Fire is the instrument of all the motion in the uni- 

 verse. Without it all bodies would become inrimove- 

 able. Men would harden into statues: and not only 

 water, but air cohere into a firm, rigid mass. 



As it is in itself, it is termed elementary fire : joined 

 with other bodies it is called culinary. The minute par- 

 ticles of this, joining with those of the pure fire, consti- 

 tute what is termed flame. Pure fire, such as is collected 

 by a burning glass, yields no flame, smoke, or ashes. In 

 itself it is imperceptible, but is discovered by its effects. 

 The first of these is heat, which arises wholly from fire, 

 and the measure of heat is always as the measure of fire. 

 E 5 



