=Steam.= The large bubbles are bubbles of steam, or water expanded by heat until its particles are so far apart that it ceases to be a liquid and becomes a gas. True steam is invisible; the moisture which collects on the sides of the tube and is seen coming out at the mouth is partially condensed steam, or watery vapor. Watch a tea-kettle as it boils on a stove; for the space of an inch or two from the end of the spout there seems to be nothing; that is where the _true_ steam is; beyond that, clouds of what is commonly called steam appear; they are watery vapor formed from the true steam by partial condensation which is produced by its contact with the cool air.[7] =Boiling-point of Water.= Water boils at different temperatures, according to the elevation above the sea-level. In Baltimore it boils practically at 212° Fahr.; at Munich in Germany at 209½°; at the city of Mexico in Mexico at 200°; and in the Himalayas, at an elevation of 18,000 feet above the level of the sea, at 180°. These differences are caused by the varying pressure of the atmosphere at these points. In Baltimore practically the whole weight of the air is to be overcome. In Mexico, 7000 feet above the sea, there are 7000 feet less of atmosphere to be resisted; consequently, less heat is required, and boiling takes place at a lower temperature. By inclosing a vessel of water in a glass bell, and exhausting the air by means of an air-pump, water may be made to boil at a temperature of 70° Fahr., showing that much of the force (heat) that is consumed in causing water to be converted into steam is required to overcome the pressure of the air. The foregoing illustrates the point that _boiling water_ is not of invariable temperature; consequently, that foods which in some places are cooked in it may in other places be cooked in water that is not boiling,--in other words, that it is not ebullition which produces the change in boiling substances, but heat. =Changes Produced in Water by Boiling.= By boiling water for a moderate time the greater part of the atmospheric gases is driven off. The flavor is much changed. We call it "flat"; but by shaking it in a carafe or other vessel so that the air can mingle with it, it will reacquire oxygen, nitrogen, and carbonic acid, and its usual flavor can thus be restored. Water which flows through soil containing lime is further changed by boiling.