VAPORIZATION. 



FIG. 3. 



outer cylinder may be six feet in diameter, and is often fifty or 

 sixty feet in length. The heated air from the fire, after tra- 

 versing the inner cylinder is conducted under the boiler by the 

 flues 0, o, before it is conveyed to the chimney. 



In locomotive steam-engines, where the principal object is 

 to generate steam in a small and compact apparatus with great 

 rapidity, a different construction is adopted. Here the boiler 



consists of two- parts, a square box 

 with a double casing, of which a 

 section is given in Figure 3, which 

 contains the fire, surrounded by 

 water in the space between the cas- 

 ings ; and a cylinder , through the 

 lower part of which pass a number 

 of copper tubes of small size, which 

 communicate at one end with the 

 fire-box, and at the other with the 

 chimney, and form a passage for the 

 heated air from the fire to the chim- 

 ney. By means of these tubes, the 

 object is accomplished of exposing 

 to a source of heat, the greatest 



O O o o 



o o o o o 



O O O O 

 o o o o 



possible quantity of surface in contact with the water. (See 

 Dr. Lardner on the Steam-Engine, Cabinet Cyclopedia}. 



The subject of distillation is a natural sequel to vaporization ; 

 but it is unnecessary to enter into much detail. The principal 

 point to be attended to is the most efficient mode of condensing 

 the vapour. Figure 1 . represents the ordinary arrangement in 



FIG. 1. 



distilling a liquid from a retort a, 

 and condensing the vapour in a 

 glass flask b, which is kept cool 

 by water dropping upon it from a 

 funnel above c. The condensing 

 flask is covered by bibulous pa- 

 per, so that the water falling upon 

 it may be made to pass equably 

 over its surface, and it is sup- 

 ported in a basin likewise con- 

 taining cold water. 



But a much superior instru- 

 ment to the condensing flask is 



