104 ANSWERS TO PRACTICAL QUESTIONS 



1 6. What philosophical principle does the solidification of 

 CO g illustrate? 



(See Philosophy^ page 242.) 



That evaporation is a cooling process. A portion of 

 the liquid CO 2 turns to vapor, and thus abstracts so much 

 heat from the remainder as to freeze it. 



1 7. Why does the division in the chimney shown in Fig. 28 

 produce two currents ? 



For a few moments there is an uncertainty a condi- 

 tion of unstable equilibrium. The heated air is endeavor- 

 ing to rise, and the cold air trying to come in to supply 

 its place. The situation of the candle in the jar deter- 

 mines the length of time before the currents start. If the 

 candle be placed on one side of the jar they will be 

 established almost instantly. 



1 8. What causes the unpleasant odor of coal-gas ? Is it 

 useful? 



Impurities which it contains. Olefiant gas has a faint 

 sweetish odor, while carbonic oxide and hydrogen, when 

 pure, are inodorous. The disagreeable smell is due in 

 part to acetylene (C 2 H 2 ). The unpleasant odor warns 

 us of the presence of coal-gas. 



19. What causes the sparkling often seen in a gas-light? 



Particles of lime taken up mechanically in the process 

 of purification. 



20. Why does H in burning give out more heat than C ? 



i Ib. of H burned in O emits heat sufficient to melt 

 315.2 Ibs. of ice; and 12 Ibs. of carbon converted into 

 CO 2 enough to melt 700 Ibs. of ice. (This subject is 



