A. Because the acid would act upon _other metals_, or upon _glass_, and prevent the experimenter's success. Q. _Which of the METALS have the GREATEST affinity to OXYGEN?_ A. Those called _potassium_ and _sodium_. Potassium and sodium derive their names from potash and soda. Potassa is the oxide of potassium; and soda is the oxide of sodium. Q. _How is the affinity of potassium and sodium for oxygen shewn?_ A. They _decompose water_ the moment they are brought into contact with it. Q. _What effect has POTASSIUM on WATER?_ A. It _catches fire_ the moment it is thrown into water, and burns with a vivid flame, which is still further increased by the combustion of _hydrogen_ separated from the water. (N.B. Water is composed of oxygen and hydrogen; and potassium separates the two gases.) Q. _What effect has SODIUM on WATER?_ A. It does _not take fire_ as potassium does, but undergoes very rapid _oxidation_. Q. _Is the FURR of KETTLES an oxide?_ A. No; the furr (or deposit of boiling water) is a precipitate of _lime and mineral salt_, separated from the water by the process of boiling. Q. _Is not this FURR of boiling water often DANGEROUS?_ A. Yes; especially in _tubular boilers_, such as those employed in railways. Q. _Why is this FURR especially TROUBLESOME in RAILWAY engines?_ A. Because it is a _bad conductor of heat_; in consequence of which, it hinders the evaporating effect of the fire, and prevents the economy of fuel. Q. _Why is this FURR especially DANGEROUS in RAILWAY engines?_ A. Because when it is deposited in the boilers, they are likely to become _over-heated_; and then _explosion_ will take place from the sudden generation of highly elastic steam. Q. _Why cannot RAILWAY engines be fed with BRACKISH WATER?_ A. Because brackish water contains _mineral salt_, which makes a much larger deposit of furr, than that which contains _only vegetable matter_. CHAPTER XIX. CARBONIC ACID GAS. Q. _What is CHOKE DAMP?_ A. _Carbonic acid gas_ accumulated at the bottom of wells and pits, which renders them noxious, and often fatal. Q. _Why is not this carbonic acid TAKEN UP by the AIR, and DIFFUSED, as it is in cities?_ A. Because (being _heavier than common air_) it cannot _rise from the well or pit_; and no wind can get to it to blow it away. Q. _Is CARBONIC ACID WHOLESOME?_ A. No; it is _fatal to animal life_, when inhaled through the mouth; acting on the stomach, _as a narcotic poison_ (i. e. a poison which produces death from drowsiness). Q. _How can any one KNOW, if a place be infested with CARBONIC ACID GAS?_ A. If a pit or well contain carbonic acid, _a candle_ (let down into it) _will be instantly extinguished_. The rule, therefore, is this--Where a _candle will burn, a man can live_; but _what will extinguish a candle_, will _also destroy life_. Q. _Why does a MINER lower a CANDLE into a mine, before he descends?_