AgTite

[L,. achates; so called from the name of the river Achates in Sicily, where it was first found.] A variety of quartz, found in loose rounded pieces in rocks, or as loose pebbles in beds of rivers or gravel. Wood may be converted into agate by infiltration with waters carrying silica in solution, as in the celebrated petrified forest of Arizona. Agates show various tints in the same specimen, and the colors are delicately arranged in stripes or bands, or blended in clouds. They take a fine polish, and are much used in the manufacture of rings, seals, beads, handles of knives and forks, cups, smelling bottles, and many other ornamental articles. Burnishers for polishing, used by bookbinders, are made of agate. in tropical America. The principal species is known as the American Aloe, or century plant, under the idea that it blooms but once in a hun¬ dred years. This is a mistake, its period of blooming being about ten years. It is called maguey by the Mexicans, who distil from its sap an intoxicating drink. The coarse thread called sisal hemp is made from the fibres of its leaves. consisting of hot and cold stages in succe ion, with an intermediate period. It comes on at fixed periods, one, two, three, or more days apart. It is now generally known as malarial fever. Tong supposed to be due to marsh miasma, it is now traced to a bacterial germ, and there is much reason to believe that the mosquito is the carrier of this germ, which it injects into man with its bite. gaseous fluid which we breathe and which sur¬ rounds the earth. It is a mixture of oxygen and nitrogen, with a small amount of carbon dioxide, the average proportions being, by volume: oxy¬ gen, 20.96 percent.; nitrogen, 79.00 per cent.; carbon dioxide, 0.04 percent. (See Atmosphere; Compre ed Air; Liquid Air.) sac found in most fish, which contains a quantity of gas, and is thought to help the fish to rise and sink in the water. It is very small in some fish, quite large in others, and wanting in sharks and some other fish. In some cases it is converted into a sort of lung and is used as a breathing organ. and engines, invented in 1869, by George Westing house, of Pittsburg. Air is compre ed by the steam of the engine boiler, and carried in tubes to the car wheels, where it exerts its pres¬ sure on the brakes when it is desired to stop the train. (See Brake.) stead of gunpowder to drive out the bullet. There is a piston to force the air into a cavity, and a valve opened by the trigger, which lets the air out against the bullet. This is driven out with some force. Air guns are of little use. air from a closed ve el, producing an empty space or vacuum in ordinary air-pumps the vacuum is very far from complete, and in the most perfect of them some air remains. Many interesting experiments may be performed in ' the exhausted receiver of an air-pump, such as the boiling of water at a much lower than the usual temperature, and the extinguishing of a candle, a proof that air is nece ary to com¬ busti on.