All highly elastic fluids are called gases.  Some are salutary, but many extremely noxious, especially such as those arising from the putrefaction of animal bodies; the burning of charcoal; corrupted air at the bottom of mines, cellars, &c.  The inflammable gas, which lights our streets, churches, shops, &c., is procured chiefly from coal, burnt in furnaces for the purpose the gas being passed through metal pipes, conveyed underground to the places where the light is required:  escaping at the orifice prepared for it, it is lighted when wanted, and burns with, a brilliant flame.  This gas consists of hydrogen and carbon; and the oxygen of the air, combined with the hydrogen, causes light as long as hydrogen and oxygen exist and combine.

     Salutary, wholesome, healthful.

     Noxious, hurtful, unwholesome.

     Putrefaction, decay.

     Orifice, opening, hole.

[Illustration:  DIAMOND CUTTING AND POLISHING.]

What is Hydrogen?

One of the most abundant principles in nature; one part of it, and eight of oxygen, form water.  It is only met with in a gaseous form; it is also very inflammable, and is the gas called the fire-damp, so often fatal to miners; it is the chief constituent of oils, fats, spirits, &c.; and is produced by the decomposition of water.

     Constituent, that which forms an essential part of
     anything.

What is Chalk?

A white fossil substance, by some reckoned a stone, but of a friable kind, which cannot, therefore, be polished as marble; by others, more properly ranked among the earths.  It is of two sorts, one a hard dry chalk, used for making lime; the other a soft, unctuous kind, used in manuring land, &c.  Chalk always contains quantities of flint-stone, and the fossil remains of shells, coral, animal bones, marine plants, &c.; from which circumstance there can be no doubt that chalk is the deposited mud of a former ocean.  The chemical name of chalk is carbonate of lime.  It effervesces strongly with an acid.

     Effervesce, to froth or foam up.

     Deposited, placed on anything.

Where is Chalk found?

In large beds or strata in the earth.  Chalk, on account of its abundance in England, forms an important feature in the scenery and geology of that country; it causes the whiteness of its sea-cliffs.  Scotland and Wales are entirely without chalk.  The white chalk is found, with interruptions, over a space above eleven hundred miles long, extending from the north of Ireland, through England, France, Belgium, Germany, Poland, and Southern Russia, to the Crimea, with a breadth of more than eight hundred miles.  The Island of Crete, now called Candia, situated in the Mediterranean, was formerly noted for its chalk.  This substance is very useful in many of the arts and manufactures.

Where is the Crimea?