This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
When ice and salt are mixed, a double action takes place: the salt makes the ice melt, and the melting ice dissolves the salt. We have already observed that heat is used up in changing matter from the solid to the liquid form (pp. 27 and 55). Melting ice and salt reach a temperature below the freezing-point of water. If we pack them around some other liquid, they draw the heat from it so fast that it freezes. This is why we use a mixture of salt and ice to freeze ice-cream.
Fill a cup with cracked ice; take the temperature of the ice with a thermometer. How cold is the ice? Mix four table-spoonfuls of ice-cream salt with the ice, and watch the thermometer. When the mercury stops falling, see what degree of cold it registers.