THE SOUTHERN PLANTER. 



525 



closed so that all communication shall be cut 

 away between the air within and that without. 

 Here, although the ceiling and walls interpose 

 to shield the people in the room from the 

 pressure of the atmosphere, still each of them 

 bears the same pressure as a person outside 

 tlie room ; and a barometric column will stand 

 as high within the room as without it. What 

 is the reason?" The reason is, that the air 

 within the room possesses the full elastic force 

 which the pressure of the atmosphere can 

 give it; the spring was compressed before the 

 room was closed, and its power of lifting the 

 barometric column is therefore the same as 

 that of the free atmosphere. 



A vessel of water, with its surface exposed, 

 yields up vapour at all temperatures, and will 

 finally disappear ; but the elastic force of this 

 vapour will depend on the temperature at 

 which it is generated, being greater the higher 

 the temperature. If the heat be sufficient to 

 boil the water, bubbles rise and sometimes 

 float for a considerable time upon the surface. 



Let us consider the cause of such a bubble, 

 whose area is one square inch. The fragile 

 thing bears the atmospheric pressure of fifteen 

 pounds. Why, then, does not the fihu burst ? 

 Simply because the elastic force of the steam 

 within the bubbles is exactly equal to the 

 elastic force of the air without ; so that the 

 film is in reality placed between two gaseous 

 cushions, which press upon it equally, in oppo- 

 site directions, and therefore neutralize each 

 other. Until the water is hot enough to pro- 

 duce steam of this tension, it cannot boil ; the 

 tendency to ebullition is subdued by the atmos- 

 pheric pressure. Under the full atmospheric 

 pressure of fifteen pounds per square inch, 

 water boils at a temperature of 212 degrees, 

 Fahrenheit; and hence steam generated at 

 this temperature is said to have an elastic force 

 equal to one atmosphere. But if a portion of 

 the atmospheric pressure be removed, water 

 will boil before it reaches 212 deg. Take, for 

 instance, the case of a bubble floating on the 

 surface of water at the top of a mountain. 



We have seen that the existence of the thin 

 film which constitutes the bubble, depends on 

 the pressure against it from within being the 

 same as the pressure upon it from without.. 

 But the pressure without the bubble on the 

 summit of the mountain is less than at the 

 surface of the sea, and hence the elastic force 

 of the steam must be less in the former posi- 

 tion than in the latter. This is the case ; and 

 to produce this feebly elastic steam less heat 

 is required; or, in other words, the boiling 

 point of water on the mountain is lower than 

 at the sea level. M 18,000 feet, on Donkia 

 mountain, in the Himalaya, Dr. Hooker found 

 that water boiled at 180 degrees ; so that tea, 

 8oup, and chocolate, which require to be made 

 with water of nearly a temperature of 212 deg. 

 Fahrenheit, would be of very inferior quality 

 in this mountain region. It is not, however, 



necessary to ascend a mountain to satisf}'' our- 

 selves that the boiling point sinks as the at- 

 mospheric pressure is diminished. If water 

 at 180 degrees be placed under the receiver of 

 an air pump, and the air be removed until the 

 pressure becomes as low as it is on Donkia, the 

 water will boil : It is not even necessary to 

 heat the more volatile liquids to produce this 

 effect. A beaker of alcohol, placed under the 

 receiver of an air pump, at the ordinary tem- 

 perature of our climate, will boif violently 

 when the receiver is sufficiently exhausted. 

 Reversing the conditions, we can by increasing 

 the pressure upon its surface, enable water to 

 attain a far higher temperature than 212 de- 

 grees without boiling. Thus, as is natural to 

 expect, an external force which resists the ten- 

 dency of heat to tear asunder the particles of 

 liquid, and convert it into vapour, elevates the 

 boiling point of the liquid. The boiling point 

 depends also on the nature of the vessel in 

 which the liquid is placed. Anything that 

 resists the separation of particles, produces the 

 same effect as an increase of external pressure. 

 Water adheres to certain surftices much more 

 strongly than to others ; thus it clings to glass 

 more tenaciously than to metal ; so that, to 

 make water boil in a glass vessel, requires 

 more heat than is necessary if it be contained 

 in a metalic vessel. 



In boiling their solutions in glass vessels, 

 chemists have to be very careful, for the adhe- 

 sion between glass and liquids is overcome by 

 jerks; instead of the amicable ebullition exhib- 

 ited in a metalic vessel, the solution boils by 

 starts; and this in sometimes so energetic a 

 manner, as to project the solution quite out of 

 the glass vessel. But the most remarkable in- 

 fluence upon the boiling point of water is ex- 

 ercised by the air which it holds in solution. 

 When water is exposed to air, a certain amount 

 of the latter is absorbed by the liquid, the mag- 

 nitude of the absorption being proportional to 

 the pressure exerted by the air on the surface 

 of the liquid. It is extremely difficult to ex- 

 pel this air, but it may be done by continued 

 boiling. Imagine a glass tube, which has been 

 exhausted by an air pump, to be half filled 

 with water which has been purged of its air 

 by boiling, and hermetically closed at both 

 ends. This water adheres so closely to the 

 sides of the tube, that v.-hen the latter is turn- 

 ed upside down the liquid will not fall down- 

 wards, but will cling to the upper portion of 

 the tube. This experiment shows that the ad- 

 j hesion between the liquid and the glass is 

 I enormously increased by the expulsion of the 

 I air, for no such elFect could be produced with 

 'ordinary water; but it also teaches us that 

 the cohesion among the particles of the liquid 

 itself is very great ; for the; e is nothing to 

 prevent the central portion of the liquid col- 

 umn from detaching itself from the portion in 

 contact with the tube, except the cohesion be- 

 tween the liquid particles themselves-. Now 



