Because the carbonic acid gas is displaced by a current of air containing _oxygen_. 306. _Why does water extinguish fire?_ Because it _saturates the fuel_, and prevents the gases thereof from combining with the oxygen of the air. 307. _As water contains oxygen, why does not the oxygen of the water support the fire?_ Because the affinity between the _hydrogen_ and _oxygen_ of the water is so strong that fire cannot separate them. Water may be decomposed by _heat_, as will be hereafter explained. But the heat of an ordinary fire is insufficient. There is, however, some reason for believing that, in cases of very large fires, such as the accidental burning of houses, &c., when the supply of water thrown upon the fire is very deficient, the water _does_ become _decomposed_, and add to the fury of the flames. 308. _Why does the blacksmith sprinkle water upon the coals of his forge?_ The blacksmith uses _small coals_ because the small pieces thereof are more easily ignited than large lumps would be, and they convey heat better by completely surrounding the articles put into the fire. He sprinkles water on the coal dust _to hold its particles together by cohesion_, until the heat forms it into a cake. A strong blast of hot hair drives the vapour of the water away, and leaves a porous mass to the action of the fire. 309. _Why, when the blacksmith thrusts a heated iron into a tankard of water, do we recognise a peculiar smell?_ Because the intense heat disengages a small volume of the gases of which water is formed. [Verse: "Oh the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out."--ROM. XI.] 310. _Which gas do we (in this instance) recognise by the smell?_ The _hydrogen_ gas. Oxygen gas possesses no odour. 311. _What is Spontaneous Combustion?_ Spontaneous combustion is that which occurs in various bodies when they become highly heated by _chemical changes_. 312. _Why is heat developed during chemical changes?_ Because, as all bodies contain _latent caloric_, the disturbance of the atoms of which those bodies are composed, during the new combinations that constitute _chemical changes_, frequently sets the caloric free, and an _accumulation of caloric_ produces spontaneous combustion. 313. _Does a match ignite spontaneously when drawn over a rough surface?_ No. Because in this case the combustion arises from heat _applied by friction_. 314. _Does phosphorous ignite spontaneously when held in a warm hand?_ Phosphorous will ignite when held in a warm hand, but it does not then produce spontaneous combustion, because it ignites through the agency of _applied heat_. 315. _But if a piece of dry phosphorous be sprinkled with powdered charcoal it will ignite, without the application of heat. Why is this?_