HEAT. 



air imprisoned in the fibres cannot form convection currents to aid the 

 transport of heat. 



The low-conducting power of air thus entangled by a loosely packed 

 solid may be illustrated by filling several test-tubes respectively with 

 wool, slag wool, loose sand, and coarse copper filings, and inserting 

 a thermometer in each. When all are plunged suddenly in boiling 

 water, there is no very great difference in the rate of 

 rise of the thermometers, for the heat comes chiefly 

 through the air in each case. 



Safety Lamp. We have already mentioned that a 

 flame does not actually come in contact with a thin 

 vessel containing water, or, rather, that the metal 

 conducts the heat away so readily that the gas in 

 contact with it is lowered below combustion point. This 

 is made use of in the Davy safety lamp. A very simple 

 experiment will serve to illustrate the principle of the 

 lamp. If a burner is placed a short distance below a 

 piece of iron gauze, and the flame is lighted, it usually 

 burns under the gauze only (as in Fig. 63A). Or, if the flame is lighted 

 above the gauze, it burns over it only (as in Fig. 63fi). The gauze is a 

 fairly good conductor, and carries the heat away rapidly to the surround- 

 ing parts, and the metal thus sharing the heat has a large surface, which 

 can radiate the supply away without rising to the combustion tempera- 

 ture. The gas in contact with the gauze on the other side from the 

 flame is thus kept from igniting. But if the gauze has not a large 

 area, or if the flame is very hot~ the supply of heat to the gauze may 

 not be got rid of with sufficient rapidity, and the gas on the other side of 

 the gauze is ignited. 



The Davy Lamp consists of a brass base containing the oil-reservoir, 



B 



FIG. 62. 



FIG. 63A. FIG. 63B. 



Showing Principle of Safety Lamp. 



and an iron gauze chamber, in which the flame burns. If fire-damp is 

 present in the air in any quantity, it burns inside the lamp over the oil 

 flame with a bluish light, but the gauze is at first able to radiate off the 

 heat, or carry it away to the body of the lamp sufficiently rapidly to 

 prevent the gas outside from rising to ignition point. If, however, the 

 flame inside the lamp gets very large, there is great danger of the lamp 

 becoming overheated. The lamp is, therefore, not by any means 

 absolutely safe, but its indications are sufficient to give warning of 

 danger, and on such warning it should be extinguished. 



