806 RETARDATION OF EBULLITION. 



heated over the lamp for some time without formation 

 of vapour. Not until the tube is strongly heated, does 

 ebullition commence ; but when it takes place, it causes 



FIG. 396 (J real size). FIG. 397 (1 real size). 



a violent agitation of the liquid, and the beginning of 

 ebullition is distinctly felt by the hand. 



Instead of using a lamp it is preferable to heat the tube by steam. 

 The tube is fixed within a lamp cylinder by means of a cork, as 

 shown in fig. 396. Steam is passed through a small tube fixed in 

 the cylinder by a perforated cork, and connected by india-rubber 

 tubing with a flask containing water. The small tube below allows 

 the steam to escape. The whole contrivance is fixed by the bend of 

 the water-hammer in the clamp of the retort-stand. The water in 

 the flask is then made to boil, and ebullition in the water-hammer 

 will not begin until the tube is considerably heated by the steam, 

 while under ordinary circumstances the heat of the hand is suffi- 

 cient to produce ebullition at the low pressure under which the 

 water -exists in the apparatus. 



In the lower cork a small groove must be cut so as to allow the 

 projecting point, near the extremity of the tube, to pass. 



The pressure on the liquid within the water-hammer 

 is so small that its boiling point is very low, and in this 

 case the retardation of ebullition takes place at a low 

 temperature. Similarly when the pressure is high, the 

 retardation under favourable circumstances takes place 

 at higher temperatures. 



