We have said the ebullition takes place at 212° Fahr. (or 100° C.), but that is only at a certain level. If we ascend 600 feet high we shall find that water will boil at a less temperature; and on the top of a mountain (say Mont Blanc) water will boil at 185° Fahr.; so at an elevation of three miles water boils at a temperature less by 27° Fahr. An increase of pressure similarly will raise the boiling point of water. The heights of mountains are often ascertained by noticing the boiling point of water on their summits, the general rule being a fall of one degree for every 530 feet elevation at medium altitudes. We append a few instances taken at random:— +--------------------+------------------+-------------+---------------+ | Place. |Height above level| Barometer | Boiling point | | | of the sea—Feet. | mean height.|of water, Fahr.| +--------------------+------------------+-------------+---------------+ | Quito | 9,541 | 20·75 | 194·2 | | Mexico | 7,471 | 22·52 | 198·1 | | St. Gothard | 6,808 | 23·07 | 199·2 | | Garonne (Pyrenees) | 4,738 | 24·96 | 203·0 | | Geneva | 1,221 | 28·54 | 209·5 | | Paris (1st floor) | 213 | 29·69 | 211·5 | | Sea level | 0 | 30·00 | 212·0 | +--------------------+------------------+-------------+---------------+ [The difference for a degree depends upon the height, varying between 510 and 590 feet, according to the elevation reached. The approximate height of a mountain can be found by multiplying 530 by the number of degrees between the boiling point and 212°. In some very elevated regions travellers have even failed to boil potatoes.] The boiling point of liquid may be altered by mixing some substance with it; and although such a substance as sawdust would not alter the boiling point of water, yet if the foreign matter be dissolved in the liquid it will alter the boiling point. Even the air dissolved in liquids alters their boiling point, and water freed from air will not boil till it is raised to a temperature much higher than 212° Fahr. Water will boil at a higher temperature in a glass vessel than in metal, because there is a greater attraction between water and glass.