No; some are more so than others, and burn with a bright flame; as wood, dry vegetables, resins, oils, fats, &c.; others with difficulty, and without any sensible flame, as soot, coal, the ashes of plants, &c. There are bodies, also, which are incombustible--that is, incapable of taking fire, as some alkalies, earths, &c. What is Caloric? Caloric is that invisible agent which produces the sensation of heat. It exists in all bodies; it is a force we are ever in want of, and thus it is hid in everything around us, and penetrates all matter, however different may be its nature or properties. What is meant by Gas? 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?