Refrigera tion 157 



Ammonia is obtained from coal, or other organic matter containing 

 nitrogen, by destructive distillation. 



Besides ammonia, sulfur dioxid, carbon dioxid, and ethyl chlorid 

 are sometimes used in refrigeration plants. 



REVIEW QUESTIONS 



1. What practical uses are there for low temperatures artificially 

 produced? 2. Describe (a) an efficient household refrigerator; 

 (6) the circulation of air within it. 3. How are foods cooled in such? 

 4. (a) What is the latent heat of fusion ? (6) How may it be shown 

 in the case of fusion of ice? (c) What use is made of it? 5. (a) Of 

 what materials is an ice cream freezer made? Why? (6) Explain 

 why cream freezes in it. 6. Explain why water is frozen by the evap- 

 oration of ether in the experiment described on page 153. 



7. What change in temperature occurs in a gas when it expands? 

 Explain and give examples. 8. What change in temperature occurs 

 in a gas when it is being compressed? Explain with examples. 

 9. Upon what facts in the behavior of ammonia does an ice-making 

 machine depend for its operation? 10. Describe fully what occurs 

 in the complete circulation of ammonia in the ice-machine. 11. What 

 is the commercial source of ammonia? 12. (a) What other gases 

 besides ammonia are used in refrigeration? (6) What are the char- 

 acteristics of each? 



SUGGESTIVE QUESTIONS 



1. Why do we not place a hot air furnace in the attic of a house, 

 and ice in the lowest part of the refrigerator? 2. Atmospheric pres- 

 sure decreases with altitude. What happens to the temperature of 

 a mass of rising air? 3. Water at the top of a lake is generally warmer 

 than at the bottom ; but the air at the top of a mountain is generally 

 colder than that at the foot. Explain. 4. It is found' that frost is 

 often formed in troublesome quantities on rock drills and other 

 machines run by compressed air. Explain why. 5. If liquid air 

 were cheap and available, what uses could be made of it in the house- 

 hold? What advantages and disadvantages would its use possess as 

 compared with ice? 6. Would a refrigerator in which the ice would 

 not melt be useful? 7. Would a refrigerator in which the ice melts 

 very rapidly be efficient? 8. Why do some people wrap ice in paper 

 or a woolen blanket ? 



