250 WELLS'S NATURAL PHILOSOPHY. 



whole water is immediately blown out of the vessel as a mist by the steam 

 formed at the same instant throughout every part of the mass. To use a 

 common expression, " the water Hashes into steam." 



Steam, like water, may be heated to any extent when con- 

 tent can Bteam fined and prevented from expanding with the increase of 

 ^e heated un- temperature ; in some of the methods lately introduced for 

 purifying oils, etc., the temperature of the steam, before its 

 application, is required to be sufficiently elevated to enable it to melt lead. 



Whatissnper- ^72. Steam which has been heated in a 

 heated Steam? geparate statc to a high degree of temperature 

 Tinder pressure, is known as " Superheated Steam." In 

 this condition its mechanical and chemical powers are 

 wonderfully increased. 



In the manufacture of lard on an extensive scale the carcass of the whole 

 hog is exposed to the action of steam at very high pressure, this acting upon 

 the mass of flesh and bones, breaks up and reduces the whole to a fat 

 fluid mass. Ordinary steam, under the same circumstances, would dissolve 

 nothing. 



Steam has also been recently applied to the carbonization of wood. For 

 this purpose ordinary steam is conducted through red hot pipes, whereby it 

 attains a very high degree of temperature. It is tlien allowed to pass into a 

 vessel containing wood intended to be converted into charcoal. The heated 

 steam, penetrating into the pores of the wood, drives off the volatile portions, 

 the water, the tar, etc., and leaves the pure carbon alone behind. 



What is High- 573. Steam generated by water boiling at a 

 pressure steam? ^q^j \^yy[^ tcmperatuie, is known as High- 

 pressure Steam. By this term we mean steam condensed 

 not by withdrawal of heat, but by pressure, just as high- 

 pressure air is merely condensed air. To obtain a double, 

 triple, or greater pressure of steam, we must have twice, 

 thrice, or more steam under the same volume. 

 What relation ^74. Thc sum of thc scusiblc heat of any 

 Mnsibie'""^nd vapor, aud the latent heat contained in it, is 

 latent heat? always the same. 



It is an established fact that the heat absorbed by vaporization is always 

 less tlie higher the temperature at wliich this vaporization takes place, and 

 just in proportion also as vapor or steam indicates a lower temperature by the 

 thermometer, it contains more latent heat. Thus, if water boils at 312°, the 

 heat absorbed in vaporization will be less by 100° than if it boiled at 212°. 

 And again, if water be boiled under a diminished pressure at 112°, the heat 

 absorbed in vaporization will be 100° more than the heat absorbed by watef 

 boiled at 212°. 



