INFLUENCE OF PRESSURE ON BOILING-POINT. 39 



temperature when the barometer is low than it does when the 

 barometer marks a higher pressure. An instrument for measuring 

 the heights of mountains is based on this principle ; the higher 

 one ascends the lower is the temperature marked by a delicate 

 thermometer placed in water made to boil by means of a spirit- 

 lamp. The influence of alteration of pressure on the boiling-point 

 of water is far greater than that on the melting-point of ice ; a 

 diminution of not far from 1 centigrade is brought about in the 

 boiling-point for every 1000 feet that we ascend. 



When salt or other soluble solid substance is dissolved in water 

 the temperature indicated by a thermometer placed in the solution 

 when boiling is always higher than that indicated similarly with 

 pure water ; but, what is curious, the vapour or steam that comes 

 off is of the same temperature, in loth cases. This may be verified 

 by means of the apparatus (fig. 19); whether the flask contain 

 pure water or pretty strong brine, the thermometer will indicate 

 the same temperature in the steam ; but if the bulb be lowered so 

 as to dip into the boiling fluid, it will be at once obvious that a 

 higher temperature is indicated with the brine than with the plain 

 water. 



It results from the opposite effects of diminished pressure and 

 presence of dissolved solid matter on the boiling-point of water 

 that people who live at high elevations above the sea (for example 

 at Quito, several thousand feet above the sea-level) have habitu- 

 ally such a rarefied atmosphere surrounding them that the tem- 

 perature of pure water boiling in an ordinary saucepan is not 

 sufficiently high to cook food in the same way as it would be done 

 at the sea-level; so that, in consequence, to raise the temperature 

 sufficiently to boil eggs, for example, a large quantity of salt must 

 be added to the water. 



The same result is attained by confining the steam of the boil- 

 ing water by using a closed boiler or digester (furnished with a 

 safety-valve) instead of an open pan ; in this way the pressure is 

 increased artificially as required, and the temperature of the boil- 

 ing water proportionately raised. In a high-pressure steam-boiler 

 (such as is used for generating motive power in factories, locomotive 

 engines, and the like) the water has to be heated a long way above 

 100 before it actually boils; thus when the pressure amounts to 

 5 atmospheres, or about 70 Ibs. per square inch, the boiling-point 

 of water is raised to about 152 instead of 100. Experiments on 

 the increased temperature required to boil water in a confined 

 space cannot be safely made without specially strong apparatus 

 provided with a safety-valve; if you attempt to boil water in 

 corked glass vessels you may be badly scalded by their bursting 



