CONDUCTIVITY. 89 



passage of heat. Without at present giving a precise significance to the 

 term, we may call this power of conducting heat " Conductivity." 



The conductivity varies enormously in different substances. We 

 may, e.g., light a wooden match, and let it burn down nearly to the 

 fingers, without receiving any appreciable quantity of heat through the 

 wood, while an iron wire held by the side of the match rapidly becomes 

 uncomfortably hot, and a copper wire can only be held for a few seconds. 

 One end of a glass-rod may be melted in a flame while the rod is held in 

 the fingers two or three inches away from the melted part, while a copper 

 rod of the same diameter, with one end in the flame, will be too hot to 

 handle at a point many inches from the flame. The difference in con- 

 ductivity may be illustrated by smearing the two rods with wax, and noting 

 the difference in the times taken by the wax to melt along the two. 



The apparent coldness to the touch of metals, as compared with other 

 solids, is explained at once by their greater conductivity. The skin is 

 generally at a higher temperature than the metal, rapid conduction 

 ensues on contact, and the hand loses much heat. If we touch a piece 

 of wood at the same temperature, the amount conducted for the same 

 slope is very much less, and the hand loses heat much more slowly. If 

 the hand, however, is colder than the metal and wood, then it receives 

 more heat from the metal, which feels much hotter than the wood. 



If we paste thin pieces of paper on to two blocks, one of wood and 

 one of iron, and hold them with the paper exposed to the flame of a 

 Bunsen burner, the piece on the wood rapidly chars, while that on the 

 iron remains unburnt. The flame may be regarded as supplying nearly 

 the same quantity of heat in each case. Though the paper is itself a 

 bad conductor, it is thin, and a comparatively small difference of tem- 

 perature between the two sides will establish a sufficient slope to carry 

 away all the heat supplied. If then, on the farther side, there is a good 

 conductor, such as iron, it will rapidly convey away all the heat supplied 

 to it, and will not allow the temperature of the side of the paper in 

 contact with it to rise very high. The other side of the paper, not being 

 very much higher in temperature, does not rise to the charring-point. 

 If, however, on the farther side of the paper, we have a block of wood, it 

 does not rapidly convey away the heat supplied, and so the temperature 

 of the side of the paper in contact with the wood rises, that of the other 

 side rising still higher, and soon the charring-point is reached. 



We may illustrate this in another way. If a thin paper cup or tray 

 is constructed so as to hold water, the water in the cup may be boiled 

 over a flame without burning the paper. The water prevents the inner 

 surface of the paper from rising above 100, while a sufficient slope will 

 be established in the paper by a not very much higher temperature for 

 the outer face, to carry through all the heat supplied by the flame, and 

 the paper, therefore, remains unburnt. If we replace the paper by a 

 thin copper vessel, the copper probably does not rise more than a fraction 

 of a degree above the temperature of the water. As an effect of this, 

 it may easily be observed that the flame does not come in contact with 

 the heated surface. The temperature of the surface remains too low 

 to permit of combustion, and there is a layer of unburnt gas, of greater 

 or less thickness, according to the temperature of the surface with 

 which it is in contact. 



