RETARDATION OF EBULLITION. 805 



gas burner. The water is not dropped in until the dish is quite 

 red hot, and only drop by drop, and not in too large a quantity. 

 The experiment is more easily made if the water used is made hot 

 to begin with, as it is then not so liable to cool the dish below 

 the temperature required for maintaining the spheroidal state. 

 When the spheroidal state is sufficiently observed, the lamp is 

 removed, and the water will soon be seen to boil very briskly. 



When a liquid, especially one free from dissolved air, is 

 slowly heated, a 4 retardation of ebullition ' may often 

 be observed; that is, the liquid rises, without boiling, 

 to a temperature which is higher than its boiling 

 point. In an open vessel this retardation is not easily 

 produced, nor is the experiment in that case free 

 from danger, because when ebullition has been retarded 

 and then at last does occur, it always takes place 

 very suddenly and with great force, which is apt to 

 shatter the vessel to pieces. The phenomenon may, 

 however, be safely observed and conveniently produced 

 by the water-hammer. The apparatus is first held in 

 a horizontal position, the bulb upwards, and the end of 

 the tube is repeatedly and very moderately knocked 

 a'gainst the side of the table, a door-post, or any other 

 fixed piece of wood. At first the sound at each knock 

 is sharp, because the water is thrown back by the elastic 

 glass wall, while at the same time a few bubbles are 

 produced. But very soon the sound becomes dull, and 

 is like that produced by knocking an empty glass against 

 the wood. The apparatus is now ready for the experiment. 

 It is then inclined until it assumes the position shown 

 in fig. 396, taking special care that no bubble of vapour 

 enters the tube, for if this happens the water in the tube 

 sinks to the level of that in the bulb, as shown in fig. 

 397. If now the tube be warmed with both hifnds no 

 vapour will be formed, and the tube may even be 



