THE EFFECTS OF HEAT. 249 



The force with which a vapor expands is called its elastic 

 force, or tension. 



The elasticity or pressure of vapors is best illustrated in the case of steam, 



which may be considered as the type of all vapors. 



"When a quantity of pure steam is confined in a close vessel, 



nerTs rheTla^ i^s elastic force will exert on every part of the interior of the 



tic force of vessel a certain pressure directed outward, having a tendency 

 steam exerted? . , , , 



to burst the vessel 



What is the When steam is generated in an open vessel its elastic forca 



Bteam fonned ™"^ ^® equal to the elastic force or pressure of the atmos- 

 iu aa open reb- phere ; otherwise the pressure of the air would prevent it from 

 forming and rising. Steam, therefore, produced from boiling wa- 

 ter at 212° F., is capable of exerting a pressure of 15 pounds upon every square 

 inch of surface, or one ton on every square foot, a force equivalent to the 

 pressure of the atmosphere. 



„ , If water be boiled under a diminished pressure, and there- 



How may the , , , , • , • , j r- 



clastic force of fore at a lower temperature, the steam which is produced irom 



Bteara be in- jj. ^jjj have a pressure which is diminished in an equal de- 

 crensed or di- '^ '■ 



minished? gree. If, on the contrary, the pressure under which water 



boUs be increased, the boiling temperature of the water and 



the pressure of the steam formed will be increased in a like proportion. TTe 



have, therefore, the following rule : — 



To what is the 571. Stcam raiscd from Water, boilitig Under 



stwrn *^^iwayl ^^J givcn prcssure, has an elasticity always 



equal? cqual to the pressure under which the water 

 boils. 



_ . , Steam of a high elastic forc3 can onlv be made in close ves- 



IIow IS steam ° 



of hi-h elastic sels, or boilers. The water in a steam-boiler, m the first in- 

 force generated? stance, boils at 212°, but the steam thus generated being 

 prevented from escaping, presses on the surface of the water equally as on 

 the surface of the boiler, and therefore the boiling point of the water becomes 

 higher and higher ; or in other words, the water has to grow constantly hot- 

 ter, in order that the steam may form. The steam thus formed has the same 

 temperature as the water which produces it. 



The temperature of the water in working steam-boilers ia 

 tent can water always much greater than 212°. It should also be borne in 



be heated un- mind that water, if subjected to sufiBcient pressure, can bo 

 oer pressure ? ■' . 



heated to any extent without boiling. There is no limit to 



the degree to which water may be heated, provided the vessel is strong 



enough to confine the vapor ; but the expansive force of steam is so enormous 



under these circumstances, as to overcome the greatest resistance which has 



ever been exerted upon it. 



If a boiler, containing water thus overheated many degrees beyond the 



boiling point, be suddenly opened, and the steam allowed to expand, th« 



11 



