FREEZING. 27 



In a somewhat similar way when solid snow or scraped ice is 

 mixed with salt the two- solids melt together as it were, and 

 form liquid brine ; the process of salting the streets is sometimes 

 used in winter time to prevent the roadways becoming too slippery 

 from snow for horses to keep their foothold. As, however, the 

 snow is thus converted into briny slush, which becomes intensely 

 chilled during the operation (Chapter XXI.), the process is of 

 somewhat doubtful benefit to pedestrians who find their boots 

 saturated with very cold salt-water which will not dry readily. 

 This peculiarity of snow and salt to become colder than ordinary 

 ice when they melt together is made use of in the preparation of 

 freezing mixtures for making ices and various other purposes. By 

 putting a thermometer first in some snow alone, and then in a 

 mixture of snow and salt, you will at once see that the latter is 

 much colder. 



Expt. 21. To Freeze Water by the Cold produced by means 

 of Snow and Salt. Get two or three pounds of snow or scraped 

 ice in a pail, preferably of wood, and then mix with it about one- 

 third or one-half of its weight of powdered salt by means of a stout 

 stick. Now place a tin can holding a tumblerful of drinking 

 water (as cold as possible to begin with) in the snow-salt slush so as 

 to immerse it nearly to the top without allowing any of the brine to 

 run into the clean water. Cover the whole up well with a blanket 

 and leave it to itself for a few minutes or a quarter of an hour ; 

 at the end of that time the drinking water inside the can will 

 be more or less completely frozen hard to a block of ice, which 

 may be extracted by removing the can from the freezing mixture 

 and gently warming the metal for a minute before the fire or by 

 dipping the can in warm water : this thaws the ice where it 

 touches the can, and so loosens the block of ice. In this, as in 

 all similar cases, the reason why the mixture of snow and salt, &c., 

 becomes cold is that during melting heat is rendered " latent " or 

 disappears (vide Chapter XXI.). 



Expt. 22. To make Ices. In order to make ordinary con- 

 fectioner's ices the arrangement employed is in substance the 

 same as 'that just described, with this difference that instead of 

 using plain water, water flavoured with lemon and sugar or mixed 

 with cream and fruit juice, &c., is employed ; and that instead of 

 leaving the liquid at rest to freeze into a block, it is kept continually 

 agitated so that the ice forms in little fragments, converting the 

 whole ultimately into a sort of thick paste. Very eatable ices 

 may be made with the simple appliances of a wooden pail to hold 

 the snow and salt, a good sized tin can to hold the liquid to be 

 frozen, and a long spoon to keep up a continuous stirring; but 



