Q. _Why is carburetted hydrogen gas frequently called MARSH GAS?_ A. Because it is generated in _meadows and marshes_ from putrefying vegetable substances. (_See ignis fatuus_, _p._ 285). Q. _What gas is evolved by the WICK of a burning CANDLE?_ A. _Carburetted hydrogen gas_: that is, the _carbon and hydrogen_ of the tallow _combine into a gas_ from the heat of the flame; and this gas is _carburetted hydrogen_, or inflammable air. Q. _Why do COAL-MINES so frequently EXPLODE?_ A. Because the _carburetted hydrogen gas_ (which is generated in these mines by the coals) explodes, when a light is incautiously introduced. Q. _How can miners SEE in the coal-pits, if they may NEVER introduce a LIGHT?_ A. Sir Humphrey Davy invented a lantern for the use of miners, called "the Safety Lamp," which may be used without danger. Q. _Who was SIR HUMPHREY DAVY?_ A. A very clever chemist, born in Cornwall. (1778--1829). Q. _What kind of thing is the SAFETY LAMP?_ A. It is a kind of lantern _covered with a fine gauze wire_, instead of glass or horn. Q. _How does this fine GAUZE WIRE prevent an EXPLOSION in the coal mine?_ A. 1st--Because flame will _never pass through fine gauze wire_: and 2ndly--Though the wire get _red-hot_, it will not ignite the gas; for carburetted hydrogen gas can be ignited only by _flame_. (N. B. The interstices of the gauze wire must not exceed the 7th of an inch in diameter.) Q. _Why will not FLAME PASS THROUGH very fine wire-GAUZE?_ A. Because the metal wire is a very _rapid conductor of heat_; and when the flame of burning gas in the lamp reaches the wire gauze, the heat (which is needful to produce flame) is _conducted away by the wire_, and the flame is _extinguished_. Q. _Does the gas of the COAL-PIT get THROUGH the wire gauze INTO the LANTERN?_ A. Yes; but the inflammable gas ignites and burns _inside the lamp_: as soon, however, as this is the case, _the miner is in danger_, and should withdraw. Q. _Why is the miner in DANGER, if the gas ignites and burns in the INSIDE of the safety-lamp?_ A. Because the heat of the burning gas will soon _destroy the wire gauze_, and then the flame (being free) will set fire to the mine. CHAPTER XXI. PHOSPHURETTED HYDROGEN GAS. Q. _From what does the very OFFENSIVE EFFLUVIA of CHURCH-YARDS arise?_ A. From a gas called PHOSPHURETTED HYDROGEN; which is _phosphorus_ combined with _hydrogen gas_. Q. _What is PHOSPHORUS?_ A. A pale amber-coloured substance, resembling wax in appearance. The word is derived from two Greek words, which mean "_to produce or carry light_." ([Greek: phôs-pherein]). Q. _How is PHOSPHORUS OBTAINED?_ A. By heating bones to a white heat; by which means the animal matter and charcoal are _consumed_, and what is left is called "_phosphate of lime_." Q. _How is PHOSPHATE OF LIME converted into PHOSPHORUS?_