164. 



HEAT. 



experiment of Fig. 85, p. 158. For if the barometer column be jacketed 

 throughout with steam, the pressure of the vapour of the water 



above the mercury just 

 depresses the top of the 

 column to the level of 

 the cistern outside, that 

 is, just exerts the atmos- 

 pheric pressure. Or if 

 a U tube A B Fig. 88, 

 closed at the top of the 

 short limb and open at 

 the other limb, be filled 

 with mercury round the 

 bend to a as in the 

 figure, and if then a 

 small quantity of liquid 

 say, for convenience 

 alcohol boiling at a lower 

 temperature than water 

 be passed round the 

 bend to the top of the 

 mercury, if the tube 

 is immersed in a water 

 bath and gradually 

 heated, the liquid be- 

 gins to evaporate when 

 its boiling-point is closely 

 approached, and when 

 ft j s j us t reached the 



FIG. 89. Reduction of Boiling-Point by 

 reduction of pressure 



mercury stands at the 

 same level in the two limbs; that is, the vapour just balances the 

 atmospheric pressure. 



If the external pressure be diminished, boiling may take place at a 

 lower than the ordinary boil- 

 ing temperature, for the 

 vapour-pressure will equal the 

 atmospheric pressure at a 

 lower temperature. This may 

 be illustrated by connecting a r. 

 flask of hot water to an air- 

 pump (Fig. 89). After a few 

 strokes of the pump, boiling 

 begins. Or, on boiling water 

 in a flask till all the air is 

 expelled, corking the flask up, 

 and holding it under a stream 

 of cold water (Fig. 90), the 

 cooling is accompanied by con- Fia " *>.-Boihng water by cooling it. 



densation, and therefore diminution of the pressure on the water and 

 rapid boiling sets in. Frequently the flask collapses in this process, 

 owing to the excess of external pressure. 



