156 Introduction to the Study of Science 



from the horns and hoofs of deer ; it was called, therefore, 

 " spirits of hartshorn " ; but it was usually obtained by the 

 destructive distillation of the horns, hoofs, and bones of cattle. 

 Other substances used in refrigeration. Ammonia is the 

 most widely utilized substance in refrigeration, but sometimes 

 sulfur dioxid, carbon dioxid, and ethyl chlorid are used in the 

 same general way. Sulfur dioxid is useful in creameries where 

 a freezing temperature is not necessary. Carbon dioxid is 

 sometimes used on ocean liners ; for, though it requires seven 

 times the pressure necessary for ammonia, it does not escape 

 and contaminate the foodstuffs in the cold-storage room. 

 Ethyl chlorid is especially useful in small refrigerators for the 

 home or market. It works under ten to twelve pounds pres- 

 sure, and gives a temperature of about 28 F. in the compart- 

 ments of a household refrigerator. Household refrigeration 

 by some such means is certain to replace the inconvenient and 

 not wholly hygienic use of ice. There is an opportunity for 

 some one with ingenuity to invent a system that will be econom- 

 ical, effective, safe, and easy to control. 



SUMMARY 



Refrigeration makes possible the cold storage and distant ship- 

 ment of perishable foods, and gives a dependable and safe ice 

 supply. 



A household refrigerator is cooled by ice, which absorbs and re- 

 moves, as it melts, 142 B. T. U. per pound. 



The heat of fusion of a substance is the amount of heat required 

 to change a pound of the substance from solid to liquid without 

 adding to its temperature. 



Ice and salt make a freezing mixture. 



The ice cream freezer illustrates the use of a freezing mixture, 

 and the relative heat conductivity of metal and wood. 



The ammonia-gas ice-making plant depends upon the difference in 

 the boiling point of the gas under different pressures. The same 

 amount of heat which is given out in condensation is absorbed in 

 vaporization. In the ice-making plant, heat is transferred from a 

 colder to a hotter body by the expenditure of energy. 



