Oxygen is the scavenger that partially cleans our gutters. It is a bird of prey that devours the offal in our fields and woods. It is nothing less than the gnawing tooth of old Father Time himself, which crumbles cities and destroys all things. Combustion, as we now know it, consists simply in the union of some combustible material with oxygen. The generic term for all this action is “oxidation.” For convenience, special names are given to particular modes. When metallic oxidation occurs we call the product “rusting.” When oxygen unites with vegetable matter we call it decaying or rotting; when with animal substances we term it rotting or putrefaction. When flame is produced, the word combustion or burning is used. The amount of heat generated is, in all cases, proportioned to the amount of chemical action. Great ingenuity and skill have been shown in the discovery and utilization of materials best calculated to combine readily with oxygen. To these, as a class, has been applied the term HYDRO-CARBONS. All substances composed essentially of the elements, hydrogen and carbon, would come under this designation. These would include coal, wood, petroleum, the fats, resins, wax and many others, with some of the gases, among which may be named light and heavy carburetted hydrogen, CH₄ and C₂H₄ respectively. [Illustration: PHOSPHORUS BURNING IN OXYGEN.[8]] In the days of our grandfathers tallow candles were almost universally employed for lighting houses, and wood for warming them. It would not be impossible to find even now, in our own country, homes illuminated (?) by a rag burning in a saucer of fat. Some of us are not too young to remember the bundle of candle-rods—nice, straight sticks used in dipping candles—snugly put away for that purpose, alas! sometimes summoned forth to assist in enforcing family discipline! [Illustration: GREEN FIRE COLORED BY A SALT OF BARIUM.[9]] Strands of twisted cotton wick were suspended from these sticks, and successively dipped into a kettle of hot tallow, until external additions made them of the requisite size. Tin candle moulds finally superseded these. Then the wick was suspended in the center and the fat poured in. In cooling, the candles contracted, and so slipped easily from the moulds. Wax candles can not be cast in moulds, as they expand in cooling. They are made by pouring successive additions upon them. They are afterward given symmetrical form by rolling and shaping. Along the sea coast I have seen women and children gathering bay berries,[10] a fruit about as large as a grain of black pepper and covered with a grayish-white, fragrant wax. When these seeds are placed in hot water the wax dissolves and serves the same purpose as tallow, making delightfully aromatic candles.