794 BOILING POINT Fio. 1. the vapor immediately above the liquid. A spirit lamp will quickly cause ebullition, the steam passing off through the open glass into a cooled receiver. (See fig. 1.) Wa- ter has been the subject of very careful experi- ments with regard to its boiling point. In consequence of the diminution of the weight of the atmosphere as we ascend to high mountain alti- tudes, the boiling point of water be- comes so low that food cannot be cooked in it. Darwin, who as- cended one of the mountains of Patagonia, was unable to cook potatoes by boiling, and various travellers have ascended heights where it was impossible to boil eggs. At the city of Mexico, which is 7,000 ft. above the level of the sea, water boils at 200 F. ; at Quito, which has an elevation of 9,000 ft, it boils at 194; and at a height of 18,000 ft. in the Himalaya moun- tains Dr. Hooker found the boiling point to be 180. In mines below the level of the sea water will not boil till it is raised to a tem- perature above 212 F. When the barometer marks 28-2 inches ebullition commences at 209, so that the time required to cook food by boiling, even in the same locality, will often vary considerably. The boiling point of water under various degrees of atmospheric pressure, and consequently at various mountain alti- tudes, may be readily obtained by placing a vessel of warm water containing a ther- mometer under the receiver of an air pump, through the top of which has been introduced a barometer. (See fig. 2.) If the water in the vessel has been raised to 212 just before beingplaced under the receiver, it will require but a stroke of the piston of the air pump to produce ebullition. By con- tinuing the exhaustion the boiling may be rendered very violent, and then the mercury in the thermom- eter will be observed to fall very rapidly. The con- version 'of the water into vapor causes the conversion of sensible into latent heat, a term which is still re- tained, although modern theory regards it as being converted into mechanical force. When the water boils at 186 F., the column of mercury in the barometer will stand at about 17'5 inches, or about the same as at the summit of Mont Blanc, at an altitude of FIG. 2. Fio. 3. about 15,700 ft. above the level of the sea. By using a large pump and a small receiver, which may be quickly exhausted, and also a small quantity of water, placed in a test tube or a vessel of that form, and some strong sul- phuric acid or chloride of calcium, for absorb- ing moisture, ebullition may be produced at a temperature as low as 45 F., or even lower. If it were possible to produce a perfect vacuum, it could be continued till the freezing point is reached ; but the cir- cumstances of the case pre- vent it. An apparatus like that represented in fig. 3 will serve to exhibit the effect of increased pressure on the boiling point. A small iron boiler, a, having a thermometer, 6, tightly adjusted, with the bulb passing to the interior, and furnished with a stopcock, c, receives at its mouth, d, a strong glass tube open at both ends, and sufficiently long to contain a column of mercury equal to the pres- sure it may be desired to produce. To the mouth a screw, through which the tube passes to near the bottom, is securely fitted. To make the experiment some mercury is poured into the boiler, and then it is about half filled with wa- ter, the bulb of the thermometer being left a little above the level. If now heat be ap- plied while the stopcock is left open, the wa- ter will commence and continue to boil at 212 F. ; but when the stopcock is closed the in- creased pressure produced by the confined steam will prevent ebullition unless the temper- ature is raised. When the mercury has been forced up the tube to a height of 30 inches, there will of course be a pressure of two at- mospheres upon the surface of the water, the boiling point of which will be raised to 249. If the heat be increased until the column at- tains a height of 90 inches, the pressure will be equal to four atmospheres, and the boiling point will be raised to 291. Eegnault, in his cele- brated experiments, used a stronger and more complex apparatus than this, and found that at a pressure of 20 atmospheres the boiling point of water was 415 '4 F. From the foregoing considerations it will be seen that a perpendic- ular column of water will have various boiling points at different depths. Thus, if a column of water is 34 ft. in height, the particles at the bottom will sustain a pressure of two atmos- pheres, and it will require the application of 249 of heat to produce ebullition at that point, and of 234 at half the depth. When steam bubbles, having a temperature much above 212, ascend through a column of liquid in a tall cylinder, they impart their excess of heat to it, and violent bursts of steam and boiling water are thrown from the mouth of the vei-
