VAPORIZATION. 49 



A difference of one degree of temperature is occasioned by an 

 ascent of 530 feet. 



When the pressure on liquids is removed by artificial means, 

 they boil at greatly reduced temperatures. This may be done by 

 placing them under the receiver of an air-pump, and exhausting. 

 When the whole air is withdrawn, liquids in general boil about 1 45 

 degrees under the temperature which they require to make them 

 boil when subject to the atmospheric pressure. In a good vacuum 

 water will boil at 67. This fact is also illustrated by a simple 

 experiment, which any one may perform. A flask containing 

 boiling water, is closed with a cork while the upper part is 

 filled with steam. The boiling in the flask may be renewed 

 by plunging it into cold water ; and the colder the water the 

 brisker will the ebullition become. But the boiling is instantly 

 checked by removing the flask from the cold water and immers- 

 ing it in very hot water. On corking the flask, the ebullition 

 ceased from the pressure exerted by the confined steam upon the 

 surface of the hot water ; but on plunging the flask into cold 

 water 3 this steam was condensed, and the water began to boil 

 under the reduced pressure. On removing the flask to the 

 hot water, the steam above ceased to be condensed, and by 

 its pressure stopped the boiling. On the other hand, in a Papin's 

 digester, which is a tight and strong kettle with a safety valve, 

 water may be raised to 3 or 400 without ebullition ; but the 

 instant that this great pressure is removed, the boiling com- 

 mences with prodigious violence. 



The facility with which liquids boil under reduced pressure is 

 frequently taken advantage of in the arts, in concentrating 

 liquors which would be injured in flavour or colour by the heat 

 necessary to boil them under the pressure of the atmosphere. 

 The late Mr. Howard applied this principle in concentrating 

 syrup of the sugar, which is apt to be browned when made 

 to boil under the usual pressure. He thus boiled syrup at 

 150 applying heat to it in a pan covered by an air-tight 

 lid, and pumping off the air and steam from the upper part 

 of the pan by means of a steam-engine. This was the most 

 essential part of his patent process, by which nearly the whole 

 of the loaf sugar consumed in this country, has been manufac- 

 tured for several years. 



In the same apparatus vegetable infusions may be inspissated, 

 or reduced to the state of extracts, for medical purposes with 



