go SCIENTIFIC RECREATIONS. 



a bad conductor it follows that the less tightly the molecules are packed the 

 less conductibility there will be ; and even a substance powdered will be a 

 worse conductor than the same substance in solid form ; and also more 

 readily in the direction of the fibres than crossways. 



Liquids do not possess great conductivity, but they, as well as gases, 

 are influenced by convection, or the transport of heat from the bottom layers 

 to the top (convehOy to carry up). We have already mentioned that the 

 heated particles of water rise to the top because they expand, and so become 

 lighter. This is convection of heat ; and by it liquids and gases, though 

 actually bad conductors, may become heated throughout to a uniform tem- 

 perature. Of course the more easily expansible the body is the more 

 rapidly will convection take place so gases are more readily affected than 

 liquids. Solids are not affected, because convection of heat depends upon 

 molecular movement or mobility, and it is obvious that the particles of solid 

 bodies are not mobile. Professor Balfour Stewart says with reference to this 

 that " were there no gravity there would be no convection," for the displace- 

 ment of the light warm particles by the heavier cold ones is due to gravity. 

 The instances of convection of heat in nature are numerous, and on a gigantic 

 scale. The ocean currents, trade winds, lake freezing, etc., while the chimney 

 draught already referred to, is another example ; and in all these cases the 

 particles of air or water are replaced by convection. In the case of the 

 lake freezing the cold particles at the top sink, and the warmer ones ascend, 

 until all the lake is at a temperature of 36'2, or say 4 above freezing. At 

 this temperature water assumes its maximum density, and then expands, as 

 IT* we have seen, instead of contracting. Ice is formed, 



and being thus lighter than water, floats ; and so unites 

 to cover in the water underneath, which is never frozen 

 solid, because the cold of the atmosphere cannot reach 

 it through the ice in time to solidify the whole mass. 



Radiant heat is the motion of heat transmitted to 

 the ether, and through it in the form of waves. The 

 sun's heat is radiant heat, and radiation may be defined 

 as " The communication of the motion of heat from the 

 articles of a heated substance to the ether." The fire gives 

 out radiant heat, and so does heated metal, and it is 

 transmitted by an unseen medium. It is quite certain 

 that the heat of a suspended red-hot poker is not com- 

 municated to the air, because it will cool equally in a 

 vacuum. Sir Humphrey Davy proved that radiant 

 heat could traverse a vacuum, for by putting tin reflectors 

 in an exhausted receiver he found that a hot substance 

 in the focus of one reflector caused an increase in the 



Fig. 84. Radiant heat. , _ . Tr , , - . 



heat of the other. If we put a red-hot or a hot sub- 

 stance in one reflector, and tinder in the other, the latter will take fire. 

 The velocity of heat rays is equal to that of light, 186,000 miles in a 



