Because it contains a large amount of water, which must be evaporated before combustion can proceed. 286. _What is the effect of this evaporation?_ A great deal of heat is unprofitably expended in driving off the water of the fuel. 287. _Why does poking a fire cause it to burn more brightly?_ Because it opens avenues through which the air may enter to supply _oxygen_. 288. _Why do "blowers" improve the draft of air through a fire?_ Because, by obstructing the passage of the current of air _over_ the fire, they cause additional air to pass _through_ it, and therefore a greater amount of _oxygen_ is carried to the coals. 289. _What is smoke?_ Unconsumed particles of _coal_, rendered volatile by heat, and driven off. 290. _What is soot?_ _Carbon_ in minute particles, driven off with other volatile matters and deposited on the walls of chimneys. 291. _Why do fresh coals increase the quantity of smoke?_ Because they contain volatile matters which are easily driven off; and because, also, they reduce momentarily the heat, so that those matters that first escape cannot be consumed. 292. _Why do charcoal and coke fires burn clearly and without flame?_ Because the _hydrogen_ has been previously driven off from those substances. 293. _Why is it difficult to light charcoal and coke fires?_ Because they contain no _hydrogen_ to produce _flame_, and assist combustion. [Verse: "He hath made his wonderful works to be remembered: the Lord is precious and full of compassion."--PSALM CXL.] A new plan of kindling fires has lately been recommended. Coals are to be laid in the _bottom_ of the fire-place to a considerable depth, then the paper and wood are to be laid on, and then a little coals and cinders over them. This plan of "laying in" the fire is precisely the _reverse_ of that which has been pursued for many years. The theory is, that when the coals in the bottom are ignited, a more even combustion is kept up, whilst the smoke and gas which would otherwise escape, and become as so much waste fuel, is burnt up, and produces heat. We have heard the plan strongly recommended by persons who have tried it, and who testify to the great economy of fuel to which it conduces. CHAPTER XV. 294. _Why does paper ignite more readily than wood?_ Because its texture is less _dense_ than that of wood; its particles are therefore more _readily heated_ and decomposed. 295. _But if articles of loose texture are bad conductors of heat, why do they so easily ignite?_ The fact that they are _bad conductors_ assists their ignition. The heat which would pass from particle to particle of the dense substance of iron, and be _conducted away_, accumulates in the interspaces of paper, and ignites it. 296. _Why does wood ignite less readily than paper?_ Because its substance is _denser_ than that of paper; it therefore requires a higher degree of heat to inflame its substance.