x HEAT 143 



This analogy between temperature and water-level is worth 

 considering more fully, as much can be learnt about the dis- 

 tinction to which the heading refers. Imagine a cylinder of 

 water ; the most accurate determination of the height at which 

 the water stands gives no information concerning the quantity 

 of the liquid contained by the cylinder ; similarly, a complete 

 knowledge of the temperature of a body tells nothing about the 

 quantity of heat which can be got out of it. Just as the water- 

 level and temperature are comparable terms, so the quantity of 

 water in a vessel can be compared with the quantity of heat in 

 a body. To ascertain the quantity of water in the cylinder, in 

 addition to knowing the height of it, the capacity of the 

 cylinder for water or its size must be known : so also to measure 

 the quantity of heat in a body, in addition to knowing its tem- 

 perature, we must also be informed of its capacity for heat. 



EXPT. 134. Arrange three glass cylinders, of different dia- 

 meters but equal heights, in a row. Pour a wine-glassful of 

 water into each of them in succession. Notice that the same 

 quantity of water fills the cylinders to different heights. The 

 level is highest in the cylinder of smallest diameter and lowest 

 in the one of largest diameter. The capacity for water of the 

 one with the greatest diameter is evidently more than in either 

 of the other cases, and consequently the same quantity of 

 water fills them to extents inversely proportional to their 

 capacities for water. 



EXPT. 135. Mix lib. of hot water with lib. of cold, and 

 observe the temperature of the mixture. This temperature 

 will be found half-way between the two original temperatures. 



EXPT. 136. Heat equal weights of lead and water in the 

 same beaker. Provide two other beakers containing equal 

 weights of cold water. Put the hot lead in one of these, the 

 hot water into the other. Stir and note the temperatures. 

 Observe carefully that the water into which the heated lead 

 is plunged is not at so high a temperature as in the other 

 case. Equal amounts of water at the same temperature are 

 thus shown to be heated to different extents by equal weights 

 of water and lead at the same high temperature. 



EXPT. 137. Mix 1 lb. of mercury at 100 C with 1 Ib. at the 

 temperature of the room. Notice that the resulting tem- 

 perature is midway between the temperatures of the two 

 quantities of mercury. 



