EBULLITION. 



313 



ous form, undergoes an immense expansion or increase of bulk, so a vapor, in 

 returning to the liquid form, undergoes a corresponding and equal diminution 

 of bulk. A cubic inch of water transformed into steam at 212°, enlarges in 

 magnitude to seventeen hundred cubic inches, as already observed. The same 

 steam, reconverted into water by abstracting from it the heat consumed in its 

 vaporization, will be restored to its former bulk, and will form one cubic inch 

 of water at 212°. Vapors raised from other bodies would undergo a similar 

 change, differing only in the degree of diminution of bulk which they would 

 suffer respectively. The diminished space into which the particles of a vapor 

 are gradually condensed when it passes into the liquid state has caused this 

 process to be called condensation* 



No liquid has been submitted to so minute an examination, with respect to 

 the effects produced upon it by heat, as water ; and, with respect to other li- 

 quids, we are compelled, in the absence of experimental proof, to reason from 

 analogy. The principle that the sum of the latent and sensible heats of vapor 

 is the same for all temperatures, may be extended, with a high degree of prob- 

 ability, to the vapors of all liquids whatever ; so that we may assume this sum 

 to be constant for each liquid, though differing in one liquid compared with 

 another. To maintain the vapor of any liquid in the aeriform state, it is there- 

 fore necessary that it should contain at least a certain quantity of heat, what- 

 ever be its temperature ; and any diminution in this quantity cannot fail to 

 produce the condensation of a corresponding portion of the vapor. If the vapor 

 of a liquid, therefore, has received no heat after having passed from the liquid 

 to the vaporous form, it cannot lose any portion of the heat it contains without 

 a partial condensation ; but it is important to observe, that a vapor, whether of 

 water or any other liquid, may, after having attained the state of vapor, receive 

 an additional supply of heat to any extent, and may thus have its temperature 

 raised to any point whatever. Independently of the heat which it received in 

 the process of vaporization, all the heat which it has thus received in the state 

 of vapor it may lose, and yet remain in that state. Under such circumstances, 

 therefore, it must not be inferred that a reduction of temperature in vapor ne- 

 cessarily causes condensation. Condensation cannot commence until the vapor 

 loses all that heat which it received after taking the form of vapor ; but when it 

 has lost so much, then any further abstraction of heat must be attended by con- 

 densation. 



By the great change of volume which a vapor undergoes in condensation, it 

 becomes an efficient means of producing a vacuum, without the exertion of 

 mechanical force. Let a glass tube be provided, having at one extremity a 

 large bulb, the other extremity being open. Let a small quantity of liquid be 

 introduced into the bulb through the tube, and let a spirit lamp be placed under 

 the bulb, so as to cause the liquid to boil. The vapor of the liquid will first 

 mix with the air in the bulb and tube ; but, as its quantity increases, its elas- 

 ticity will cause it to issue through the tube, which it will at length raise to 

 its own temperature, so as to enable it to pass from the mouth of the tube in 

 the vaporous form, without being previously condensed. The stream of vapor 

 proceeding up the tube will, after a time, carry off with it the atmospheric air pre- 

 viously contained in the bulb and tube ; and at length the space below the mouth 

 of the tube will be completely filled with pure vapor. Let the tube be now 

 inverted, and its open end plunged in a vessel of water or .other liquid, the 

 bulb being presented upward. The space within the tube and bulb containing 



* In general, whenever the dimensions of a body are diminished, without any diminution of its 

 quantity of matter, it is said to be condensed, and the process may without impropriety be called 

 condensation, ; but this more general application of the term cannot cause any confusion, since its 

 meaning is always easily understood from the context. 



