QUANTITY AND INTENSITY. 7 



strokes on a large mass of iron will produce but very 

 slight change of temperature. The quantity of muscular 

 energy expended is the same in each case, but the inten- 

 sity of heat energy produced varies inversely as the 

 mass. 



The intensity varies also as the resistance. A small 

 piece of wood held in the flame of a lamp is quickly 

 ignited at the end in the flame ; while the end held in 

 the hand shows no perceptible change of temperature. 

 But a brass rod of the same size, similarly held for the 

 same length of time, becomes too hot for the hand long 

 before the end in the flame is hot enough to ignite 

 wood. 



In the wood, the intensity rises rapidly at the end 

 held in the flame, because the resistance prevents dis- 

 tribution of heat through the mass. But the low 

 resistance of the brass permits the rapid distribution 

 of the same quantity of heat through its mass ; so that the 

 intensity at the end in the flame is much less than that 

 of the wood. 



In kindling a fire of anthracite coal, when the pro- 

 portion of coal is too great for the kindling-wood, the 

 heat generated by the consumption of the wood fails to 

 ignite the coal, because such coal being a comparatively 

 good conductor of heat, the amount is rapidly distrib- 

 uted through the mass, and hence the intensity at any 

 point is insufficient to produce ignition. But if the pro- 

 portion of coal be sufficiently reduced, the consumption 

 of the same amount of wood will produce ignition. 

 The quantity of heat imparted to the coal is the same in 

 each case, but its intensity is greater in the latter case. 



In electric experiments there is a great difference 

 noticeable in the amount of work required to produce 



