66 ANSWERS TO PRACTICAL QUESTIONS 



31. Why is it beneficial to stir a wood-fire, but not one 

 of anthracite coal? 



The gaseous constituents of wood are more easily separated 

 than those of coal, but the heat evolved in combustion is less. 

 When once well started, the combustion of the coal is hence 

 more apt to be self-sustaining. 



32. Why ivill water put out a fire ? 



It absorbs more heat in proportion to its weight than any 

 other known substance does in changing from liquid to gas. 

 The production of steam is hence at the expense of the heat of 

 combustion, and the fuel is soon cooled below its kindling point. 



33. What should we do if a person's clothes talce fire ? 



Smother the flame by wrapping the person as quickly as 

 possible with a rug, coat, blanket, or any thing of this kind that 

 may be at hand. If water be near, throw it abundantly on 

 the burning garment. . . 



34. Ought the doors of a burning house to be thrown 

 open? 



No, except for the purpose of getting out of it. The in- 

 creased supply of air through the opening promotes the com- 

 bustion. 



35. Hoiv much O can be obtained from 1OO grams of 



Iff/0? 



HgO : O : : 100 : x. 



216 : 16 : : 100 : x. 

 x = 7.4+, grams. 



30. What would be the volume of the O of Question 35 

 under the standard conditions ? 



7.4-*- 1.43 = 5.17 liters. 



37. JVJiat would be the volume of the O at 12 C. and 

 under a pressure of 74O mm. of mercury ? 



x = * x 5 - 17 = 5 - 42 + ' liters - 



