796 PRESSURE OF STEAM. 



to the pressure of the atmosphere ; at a lower tempera- 

 ture the pressure of steam is less, at a higher tempera- 

 ture it is greater than that of the atmosphere. A 

 liquid in an open vessel cannot be heated to a higher 

 temperature than its boiling point, for any addition of 

 heat after the liquid has reached its boiling point is 

 employed in the formation of vapour. Any cause, 

 however, which prevents the escape of steam makes 

 the temperature rise. If, for instance, water be boiled 

 in a closed vessel, the water may be heated to any tem- 

 perature, if the vessel be strong enough. The higher 

 the temperature rises the greater becomes the pressure 

 of the vapour, and if the vessel is not strong enough to 

 resist the increasing pressure it very soon bursts with 

 a loud report. 



It is not advisable to attempt an experiment on the pressure of 

 steam in a closed vessel of larger dimensions than 

 that shown in fig. 391. Little glass bulbs of this size, 

 already filled with liquid, are sold at the dealers in scien- 

 tific apparatus by the name * candle bombs.' When 

 stuck in a lighted candle against the flame, or hung 

 FIG. 391 by a wire into a gas-flame, they soon explode. With 

 a size). jaeger vessels most serious accidents may be caused by 

 the explosion. 



The pressure of vapour of water heated beyond 

 100 is used on a large scale for giving motion to 

 steam-engines. 



If a vessel containing water is heated, and there is 

 only a small orifice in the vessel, then the escape of the 

 steam is impeded, and the temperature of the water 

 rises to above 100. The tension of the steam is in 

 that case higher than the atmospheric pressure, and the 

 steam is ejected through the orifice with considerable 



