think out the reason of this statement for yourself, but if you can not 

 you will understand the matter more fully when you read the next 

 section. What we have seen from the former of these experiments, 

 namely, that water is a poor conductor, is true of all liquids. 



As to the conducting power of gases, it is so yery difficult a 

 thing to test by experiment that you must for the present be 

 satisfied to know that gases are very poor conductors indeed. 

 In fact, the warmth of woolen or flannel clothes is mostly due to the 

 fact that air clings to the threads and in the openings between threads, 

 and it is the very poor conducting power of the air that prevents the 

 heat of the body escaping. You will now be able to explain why ice 

 houses are often built with double walls having an air space between 

 the walls, and why in cold climates outside doors and windows are put 

 on in the winter time. You will no doubt think of many other ex- 

 amples, if you try. 



HOT CURRENTS AND COLD CURRENTS. 



There is another way in which heat travels in liquids and gases, 

 namely, by the formation of currents that carry the heat with them. 

 This is the way in which water in a kettle is heated by the fire. You 

 may see this process, which is called convection, by the following 

 experiment: Take an ink bottle in which about a spoonful of ink 

 has been left. Warm it up gradually by dipping it a few times into 

 a vessel of warm water. Then fill it with hot water and put it at the 

 bottom of a pail fidl of cold water. The inky hot water will come out 

 of the bottle and j-ou can see the way in which this hot water rises 

 through the midst of the cold water. Now, that is the way in which 

 water moves in a pot or kettle that is being heated on the stove. But 

 it is evident that as the hot water rises in the middle of the pail, cooler 

 water must go down along the sides, and so we have hot and cool cur- 

 rents moving up and down and conveying heat to all parts of the pot. 

 Here is an interesting illustration which you may try for yourself. 

 Make a box out of thick writing paper by folding it at the comers and 

 stitching the flaps with thread to keep them from opening. Then 

 hang it up by threads attached to the four corners, half fill it with 

 water and bring a flame beneath it. You will find that the water can 

 be boiled without scorching the paper. This shows how rapidly heat 

 is carried off by convection. Currents of hot water rise to the surface 

 conveying heat away and cold currents descend to take the place of 

 the hot water that has risen. Thus the bottom of the box is kept 

 cool. If it were not for this action the paper would burn. In fact, 

 if the fiarae is too big the paper does get scorched above the water 

 line. 



