EVAPORATION 



309 



4-92 inches of mercury, or 2-42 lbs. per sq. in., provided that the atmospheric 

 pressure is 29 • 92 ins. Conversely, when ebullition occurs at pressures above 

 atmospheric the liquid is said to boil under pressure, such pressures being 

 usually reckoned from the normal atmospheric pressure as zero. Thus, 

 steam at 30 lbs. pressure or 30 lbs. gauge means that the pressure is 30 lbs. 

 per sq. in. above that, due to the atmosphere, corresponding to an absolute 

 pressure of 44-707 lbs. per sq. in. At the end of this chapter will be found 

 a table connecting the boiling points of water with the pressure under which 

 ebullition occurs. 



The boiling point of a solvent is elevated by the presence of solids in 

 solution. For non-electrolytes, as sugar, and in dilute solutions generally 

 the elevation is proportional to the quantity of material in solution in unit 

 volume, and molecular weights of different non-electrolytes give the same 

 elevation of the boiling point. This elevation of the boiling point is inde- 

 pendent of the pressure under which ebullition occurs. In the annexed 

 table the values from 10 to 70 per cent, are due to Gerlach, the balance being 

 after Claassen.* 



Table giving the Boiling Point of Sugar Solutions. 



If the vapour from a liquid be mixed with the vapour from a second 

 liquid, or with an incondensihle gas, the pressure exerted is the sum of the 

 individual pressures. Thus, if the pressure of water vapour in an enclosed 

 space is found to be higher than that which corresponds to the temperature, 

 the presence of air or other gas is indicated. 



* The boiling points of sugar solutions were first given by Dutione in i/QO, and were determined by him as 

 a guide to the operation of sugar boiling in the open train. Generally, the conduct of sugar boiling has not yet 

 reached this degree of refinement. 



