326 



VAPORIZA TION. 



I 



510. Definition of Boiling Point. We ought 

 now to be fully prepared to understand that the boiling 

 point of a liquid is the temperature at which it 



gives off a vapor of the same 

 tension as the surrounding at- 

 mosphere. 



(a.) If there be any doubt or lack of 

 comprehension of this proposition, it may 

 be removed by the following experiment : 

 A. A glass tube, bent as shown at A, fcas its 

 short arm closed and its long arm open. 

 The short arm is nearly filled with mer- 

 cury, the space above the mercury being 

 filled with water. While water is briskly 

 boiling in a flask, the bent tube is sus- 

 pended in the steam, as shown in Fig. 

 250. Part of the water in the bent tube 

 is changed to vapor, the mercury falls in 

 the short arm, and finally assumes the same 

 FIG. 250. level in both branches. 



511. Distillation. Distillation is a process of sep- 

 arating a liquid from a solid which it holds in solution, or 

 of separating a mixture of two liquids having different 

 boiling points. The process depends upon the fact that 

 different substances are vaporized at different temperatures. 

 The apparatus, called a still, is made in many forms, but 

 consists essentially of two parts the retort for producing 

 vaporization, and a condenser for changing the vapor back 

 to the liquid form. Fig. 251 represents one form of the 

 apparatus. It consists of a retort, ab, the neck of which is 

 connected with a spiral tube, dd, called the worm. The 

 worm is placed in a vessel containing water. This vessel 

 is continually fed with cold water carried to the bottom by 

 the tube h. As the water is warmed by the worm it rises 

 and overflows at i. 



