160 SCIENCE OF COMMON THINGS. 



Milk boila qv.lcker than water. Why stoves are not placed at top of the room. 



agitation by the violence of the convective currents) are 

 made visible by the strong beam of light thrown into 

 the room through the crevice of the shutter. 



1O37* Why does milk boil more quickly than tvater? 



Milk is a thicker liquid than water, and consequently 

 less steam escapes through the thick liquid (milk) than 

 through the thin liquid (water) ; therefore the heat of 

 the whole mass of the milk rises more quickly. 



1O3Q Why are fires placed near the floor of a room, and not towards 

 the ceiling ? 



Because heated air always ascends. If, therefore, 

 the fire were not near the floor, the air of the lower 

 part of the room would be elevated in temperature very 

 slowly. 



1039 Would not the air of the lower part of a room be heated equally 

 well if the fires were fixed higher up f 



No ; the heat of a fire has a very little effect upon 

 the air 'below the level of the fire / and therefore every 

 fire should be as near to the floor possible. 



1040 Our feet are very frequently cold when we sit close by a good fire : 

 explain the reason of this ? 



As the air over the fire becomes heated and rises, 

 cold air rushes through the crevices of the doors and 

 windows, and along the bottom of the room, to supply 

 the deficiency ; and these currents of cold air rushing 

 constantly over our feet, deprive them of their warmth. 



104:1 What is meant by radiation of heat f 



The emission of rays of heat in all directions. 



"When the hand is placed near a hot body suspended in the air, a sen- 

 sation of warmth is perceived, even for a considerable distance. If the 

 hand be held beneath the body, the sensation will be as great as upon 

 the sides, although the heat has to shoot down through an opposing cur- 

 rent of air approaching it. This effect does not arise from the heat being 

 conveyed by means of a hot current, since all the heated particles have a 

 uniform tendencj^ to rise; neither can it depend upon the conducting 

 power of the air, because aerial substances possess that power in a very 

 low degree, while the sensation in the present case is excited almost on 

 the instant. This method of distributing heat, to distinguish it from heat 

 passing by contact or conduction, is called radiation. 



1O4S How do we designate heat so distributed f 



As radiant or radiated heat. 



