Transportation on Land 303 



heat is present in any substance which has a temperature above 

 absolute zero or 273 C. If pressure is reduced low enough, 

 any volatile substance will boil at a correspondingly low tem- 

 perature (see page 312), and its steam will have the same temper- 

 ature as that of the boiling liquid under that pressure. 



Some industrial uses of low boiling temperature. Many in- 

 dustries make use of the fact that the boiling temperature of a 

 liquid can be lowered by the reduction of the pressure upon its 

 free surface. If such substances as milk, eggs, sugar sap, and 

 cider were boiled at ordinary pressure, they might be easily 

 scorched or otherwise damaged. They are usually evaporated 

 under low pressure or in vacuum pans at relatively low boiling 

 temperatures. The fact is turned to advantage also in econ- 

 omizing fuel. In the manufacture of glycerine, for example, 

 from the waste lye solution from soap making, the evaporation 

 is carried on at different pressures and temperatures. The 

 steam produced from the first evaporator is conducted to and 

 boils the liquid in a second evaporator which is kept at a suffi- 

 ciently lower pressure. The steam from this is likewise made 

 to boil a third, and so on possibly to a fourth evaporator. Thus 

 the latent heat of steam is utilized three or four times, saving 

 almost two thirds or three fourths of the fuel required for 

 evaporation at normal atmospheric pressure. 



The chief points presented in these experiments and observa- 

 tions may be summarized briefly. 



First. The boiling temperature of water remains the same 

 under the same general conditions. 



Second. The temperature of a boiling liquid remains the 

 same, if pressure is constant, until all of the liquid is vaporized. 



Third. The boiling temperature of a liquid varies with 

 pressure ; rising or falling as pressure increases or decreases. 

 Pressure may be controlled mechanically, and thereby the 

 boiling point as well. 



Fourth. The boiling temperature of a liquid is that at which 

 the pressure of its saturated vapor, produced at the place of 



