FREEZING. 85 



cool water in porous jars, which permit some of it to pass off in vapour, 

 the remainder being cooled. 



Sir John Leslie invented a method of freezing water by rapid evapora- 

 tion on sulphuric acid under the receiver of an air-pump, and water has 

 been frozen even on a hot plate by these means. By pouring sulphurous 

 acid and water on this plate, the acid evaporates so quickly that it produces 

 sufficient cold to freeze the water it quitted into solid ice. 



We leave the phenomena of clouds and watery vapour in the atmo- 

 sphere for consideration on another opportunity, under the head of Meteor- 

 ology y Raiu t etc. 



Fig. 80. Apparatus for freezing carafes of water. 



An experiment is often performed by which water is frozen in a vacuum. 

 By putting a saucer full of water under the receiver of an air-pump it will 

 first boil, and then become a solid mass of ice. It is not difficult to under- 

 stand the cause of this. The water boils as soon as the air is removed ; but 

 in order to pass from the liquid to the gaseous state without the assistance 

 of exterior heat, it gives out heat to the surroundings, and in so doing 

 becomes ice itself. This fact Mr. Carre has made use of in the apparatus 

 shown above (fig. 80). A small pump creates a vacuum in the water bottles, 

 and ice is formed in them. 



This apparatus might easily be adopted in country houses, and in places 

 where ice is difficult to procure in summer. The only inconvenience attend- 

 ing it is the employment of sulphuric acid, of which a considerable quantity is 



