THE BOILING POINT. 803 



in consequence of the diminished pressure the water in 

 the retort soon begins to boil. 



If the tube of the water-hammer be held between both 

 hands in the position shown in fig. 394, the warmth of 

 the hands will be sufficient to cause a rapid evapora- 

 tion of the water which remains adhering to the glass. 

 The vapour formed passes in bubbles through the water, 

 but is soon condensed again in consequence of the low 

 temperature of the water. 



The following small table contains the boiling point 

 of some substances at a pressure of 760 mm ': 



Sulphur . 448 C 



Mercury . . . 350 C 

 Sulphuric Acid . . 323 



Alcohol . . . 78-4 

 Ether . . . 34'9 



Sulphurous Acid . 10 '8 



If alcohol or ether be used as the liquid in an 

 apparatus similar to the water-hammer, ebullition is 

 always much more brisk than in the water-hammer, 

 because the vapour of alcohol and ether have a much 

 higher tension than the vapour of water has at the 

 same temperature. 



When a small quantity of liquid is placed upon a 

 surface which is considerably hotter than the tempera- 

 ture of the boiling point of the liquid, the evaporation 

 presents remarkable phenomena. A small dish of 

 copper or platinum is heated to redness, and a little 

 water is dropped into it by means of a pipette or wash- 

 ing-bottle, as in fig. 395. Under these circumstances, 

 the liquid does not spread itself out on the dish, it does 

 not moisten it, and produces no hissing sound, as it 

 would if placed upon a moderately hot surface, but it 

 assumes the form of a flattened globule, like a drop of 

 mercury. If the quantity of water is small, it re- 



