90 HEAT. 



In steam boilers we have a somewhat similar effect. A temperature 

 slope is established from the flame to the water within the boiler, but 

 the outside temperature of the metal may be not greatly above that 

 of the water, if the thickness is not very great. If, however, there is 

 a badly conducting incrustation within the boiler, the conditions are 

 altered. In order that the water may receive the same quantity of 

 heat, the outer side of the incrustation must be much hotter than the 

 inner ; the metal must therefore be much hotter than the water. It 

 may therefore rise to the temperature at which it is easily burned by 

 the flame, or it may even be softened through heat so much as to give 

 way to the internal pressure. 



Another effect of the incrustation may be noted. In order to get 

 the same quantity of heat into the water through the badly conducting 

 wall, a higher temperature may be necessary within the furnace, and 

 then there will be a greater quantity of heat passing through every 

 outlet at which there is waste. 



On the other hand, if we surround the outside of a boiler with a 

 badly conducting layer of boiler " clothing " to keep its heat in, a very 

 large difference of temperature between the two sides of the layer 

 will correspond to only a small flow of heat from the boiler, so that 

 though the inside may be at the temperature of the water, and the 

 outside at that of the air, no very great quantity of heat escapes. 



Of course, the same principle is used in our own clothing. We put 

 on non-conducting substances of such thickness and quality that the 

 temperature slope from our bodies to the outer air shall not carry off 

 more heat than we are willing to part with. 



General Remarks on Conductivity in the Three States. 

 Common observation tells us that solids differ enormously in their con- 

 ductivity, the metals being the best conductors. They range from copper 

 and silver, which are the best, downwards through minerals and wood to 

 furs and feathers, which are probably the worst. 



Liquids are Bad Conductors. Though we may readily heat liquids 

 they are in general bad conductors. When we boil water in a vessel, 

 we always supply heat from below, so that the heated water may rise 

 and give place to colder water from above, which is heated in its turn. 

 The hot water rising, comes in contact with much colder water, so that 

 the temperature-slope is very steep, and the heat is soon shared between 

 the hotter and colder portions, even though the liquid is a bad conductor. 

 This process termed " convection " is therefore really only conduction 

 aided by a transportation of the hotter matter as it were, an artificial 

 deepening of the temperature-slope. If, instead of heating the water 

 from below, we heat it from above, then it takes a very much greater 

 time to get heated throughout. It is quite easy to boil water on the 

 top of a test-tube, holding the bottom part in the hand, and receiving 

 no sensible amount of heat. The experiment is still more striking if a 

 small quantity of ice is kept at the bottom of the tube by a little wire 

 gauze (Fig. 61). 



If convection is prevented, we find that liquids are generally to 

 be ranked with the worse conducting solids. 



Gases are also bad conductors, worse even than liquids. Here again 

 the badness of conduction is masked by convection. The air gets hot 



