THE SCIENCE OF THE KITCHEN 105 



that contains, the substance that is to be frozen. It consists 

 of ice and salt, about three parts of ice to one of salt. Salt 

 has so great an affinity for water that when it is placed in 

 contact with ice it causes the ice to melt. It requires heat 

 to melt ice, and thus heat is taken from the surrounding 

 bodies. A temperature may be obtained as low as 25 or 

 30 degrees below the freezing point, that is, almost down to 

 zero Fahrenheit. The brine formed does not freeze because 

 its freezing point is much lower than that of pure water. 



LABORATORY EXERCISE 14 



Purpose. To study the principles involved in freezing ice 

 cream. 



Materials. Salt, ice, tumbler, thermometer, test tube, tin 

 cup. 



Directions, i. Put a mixture of salt and crushed ice in a 

 tumbler. Put in this the bulb of a thermometer and record the 

 temperature. Try different proportions of ice and salt and find 

 which gives the lowest temperature. 



2. Put a test tube containing water into the mixture of ice 

 and salt. 



3. Fill a tin cup with snow. Place the cup on a board 

 covered with a layer of water. Stir some salt into the snow. 

 What happens to the water on the board and bottom of the 

 cup ? Why ? 



Causes of decay of foods. One common problem in the 

 home is to keep foods from decaying. The decay of foods 

 is caused chiefly by two kinds of small plants, molds and 

 bacteria. Molds may be seen with the naked eye as thread- 

 like plants growing upon various kinds of food. On these 

 threads appear little stalks, each bearing a ball. These are 

 filled with a powder called spores, which scatter and grow 

 into new plants. These molds have no green coloring matter 

 and so cannot make their own food like ordinary plants, 

 but must live on food already made by other means. 



