ATMOSPHERIC AHl. 133 



into steam. We have reason to believe that every solid sub- 

 stance on the face of the earth might be converted to a fluid, 

 or even to a vapour or gas, were it submitted to the action 

 of a very high temperature in peculiar circumstances. Some 

 bodies give out their superabundant caloric much sooner 

 than others. Iron is a quicker conductor of caloric than 

 glass, and glass than wood. If you take a piece of iron in 

 one hand, and a piece of wood in the other, the iron feels 

 cold, the wood warmer, though the thermometer shows that 

 their temperature is the same. Substances usually become 

 more dense by the loss of caloric ; but the freezing of water 

 is a striking cxcejition to this general law of nature, and is a 

 memorable instance of the wisdom and provident care of the 

 Almighty, when he established the laws of the universe. 



QuKSTioNs. — 1. What is heat? 2. Why is the matter of heat 

 called caloric ? 3, How are sensations of heat and cold produced ? 

 4. What is cold ? 5. What is sensible caloric ? 6. Latent caloric ? 7. 

 What experiment illustrates this ? 8. Why is one body said to have 

 a greater capacity lor caloric tlian another ? 9. How do bodies lose 

 their capacity for caloric ? 10. Why is caloric evolved during the 

 slaking of quick-lime ? 11. When is a capacity for caloric increased . 

 12. Describe the experiment. 13. What is specific caloric ? 14. Of 

 what use is a thermometer ? 15. Of what does it consist .'* 16. How- 

 is it constructed ? 17. How is caloric the cause of fluidity .'' 18. What 

 is said of conductors of caloric ? 19. To what general law of nature 

 is the freezing of water an exception .-' 20. What are the difl'ereni 

 kinds of thermometers ? (See Appendix.) 21. How is each gra- 

 duated ? 



LESSON 61. 



Atmospheric Air. 



Gas. When solid substances are rendered permanently aSriform 

 by heat, the air, thus produced, is called a gas. All the gases 

 are compounds of solid matter and caloric. It is caloric which 

 separates the particles, and gives to the whole a gaseous form. 

 The permanency of the gases appears to be owing to the 

 strength of the affinity existing between caloric and their bases, 

 which affinity resists every reduction of temperature. 



The atmosphere, which was formerly supposed to be a 



simple fluid, is composed of two distinct substances, termed 



oxygen gas and nitrogen gas. It is not a chemical com,' 



pound, but a mere mixture of those gaseous substances in 



12 



