VAPORIZATION. 



with great rapidity, till their temperature rises to within a de- 

 gree or two of 212. For some purposes, a pan containing the 

 matters to be heated is placed within another and similar larger 

 one,, and steam admitted between the two vessels. Manufac- 

 tured goods also are often dried by passing them once over a 

 series of metallic cylinders, or square boxes filled with steam. 

 Factories are now very generally heated by steam, conveyed 

 through them in cast iron pipes. It has been found by practice 

 that the boiler to produce steam for this purpose, must have 

 one cubic foot of capacity for every 2000 cubic feet of space to 

 be heated to a temperature of 70 or 80 ; and that of the con- 

 ducting steam pipe, one square foot of surface must be exposed 

 for every 200 cubic feet of space to be heated. 



The expansion of water into steam is used as a moving power 

 in the steam engine. The application is made upon two different 

 principles, both of which may be illustrated by the little in- 

 strument depicted on the margin. It consists of a glass tube, 

 about an inch in diameter, slightly expand- 

 ed into a bulbous form at one extremity, 

 and open at the other ; a piston is made, by 

 twisting tow about the end of a piece of 

 straight wire, which must be fitted tightly 

 in the tube by the use of grease. Upon 

 heating a little water in the bulb below the 

 piston, steam is generated, which raises the 

 piston to the top of the cylinder. Here 

 the simple elastic force of the steam is the 

 moving power ; and in this manner steam 

 is employed in the high pressure engine. The greater the load 

 upon the piston, and the more the steam is confined, the greater 

 does its elastic force become. Again, the piston being at the 

 top of the cylinder, if we condense the steam with which the 

 cylinder is filled, by plunging the bulb in cold water, a va- 

 cuum is produced below the piston, which is now forced down 

 to the bottom of the cylinder by the pressure of the atmos- 

 phere. In this second part of the experiment, the power is 

 acquired by the condensation of the steam, or the production 

 of a vacuum ; and this is the principle of the common con- 

 densing engine. In the first efficient form of the condensing 

 engine (that of Newcomen) the steam was condensed by inject- 

 ing a little cold water below the piston, which then descended, 

 from the pressure of the atmosphere upon its upper surface^ 



