307. Natural Gas occurs near Pittsburg, Pa., and in many other places, in immense quantities. It is not only employed to light the streets and houses, but is used for fires and in iron and glass manufactories. It is estimated that 600,000,000 cubic feet are burned, saving 10,000 tons of coal daily in Pittsburg, Only half a dozen factories now use coal. More than half the gas is wasted through safety valves, on account of the great pressure on the pipes as it issues from the earth. These reservoirs of natural gas very frequently occur in sandstone, usually in the vicinity of coal-beds, but sometimes remote from them. In all cases the origin of the gas is thought to be in the destructive distillation, extending through long geological periods, of coal or of other vegetable or animal matter in the earth's interior. Natural gas varies in composition, and even in the same well, from day to day; it consists chiefly of CH4, with some other hydro-carbons. CHAPTER LVI. ALCOHOL. 308. Fermented Liquor. Experiment 130.--Introduce 20 cc.of molasses into a flask of 200 cc, fill it with water to the neck, and put in half a cake of yeast. Fit to this a d.t., and pass the end of it into a t.t. holding a clear solution of lime water. Leave in a warm place for two or three days. Then look for a turbidity in the lime water, and account for it. See whether the liquid in the flask is sweet. The sugar should be changed to alcohol and CO2. This is fermented liquor; it contains a small percentage of alcohol. 309. Distilled Liquor. Experiment 131.--Attach the flask used in the last experiment to the apparatus for distilling water (Fig. 32), and distil not more than one-fifth of the liquid, leaving the rest in the flask. The greater part of the alcohol will pass over. To obtain it all, at least half of the liquid must be distilled; what passes over towards the last is mostly water. Taste and smell the distillate. Put some into an e.d. and touch a lighted match to it. If it does not burn, redistil half of the distillate and try to ignite the product. Try the combustibility of commercial alcohol; of Jamaica ginger, or of any other liquid known to contain alcohol. 310. Effect on the System. Experiment 132.--Put a little of the white of egg into an e.d. or a beaker; cover it with strong alcohol and note the effect. Strong alcohol has the same coagulating action on the brain and on the tissues generally, when taken into the system, absorbing water from them, hardening them, and contracting them in bulk. 311. Affinity for Water. Experiment 133.--To show the contraction in mixing alcohol and water, measure exactly 5cc.of alcohol and 5cc.of water. Pour them together, and presently measure the mixture. The volume is diminished. A strip of parchment soaked in water till it is limp, then dipped into strong alcohol, becomes again stiff, owing to the attraction of alcohol for water.