144 ELEMENTARY GENERAL SCIENCE CHAP. 



EXPT. 138. Mix 1 Ib. of water at the temperature of the 

 air with 1 Ib. of iron at 100 C, and notice the temperature of 

 the mixture. Mix 1 Ib. of water at 10(F C with 1 Ib. of iron at 

 the atmospheric temperature, and notice that the resulting 

 temperature is in the latter case much the higher. 



EXPT. 139. Shake up known weights of water and mercury 

 at different temperatures, and note the resulting temperatures. 

 Repeat, using water and turpentine at known but different 

 temperatures. Confirm by shaking up mercury and tur- 

 pentine. The relative capacities for heat are inversely as the 

 weights, and inversely as the change of temperature. 



Measurement of Quantities of Heat. As in all other cases 

 of measurement, we must have some unit in terms of which 

 to compute the quantity of heat which is being measured. 

 The unit of heat generally adopted is the quantity of heat 

 necessary to raise the temperature of 1 gram of water 

 through 1 C. In terms of this unit the amount necessary to 

 raise 10 grams of water through 1 would be 10 units of heat, 

 or that required to raise 1 gram through 10 would be .the 

 same amount. Water has the greatest capacity for heat of all 

 forms of matter ; in other words, the quantity of heat required 

 to raise 1 gram of water through 1 C. is greater than the 

 quantity required to raise 1 gram of "any other substance 

 through the same interval of temperature. 



As we have defined the unit quantity of heat, the capacity 

 which any substance possesses for receiving heat may be ex- 

 pressed in terms of the unit. The capacity for heat of a body 

 is the number of units of heat required to heat it through 

 1 G C. The capacity for heat (or thermal capacity) of unit 

 mass is called the Specific Heat. 



As water has the highest capacity for heat, any weight of 

 water at any temperature contains more heat than the same 

 weight of any other substance at the same temperature. A pound 

 of water at 100 C would thus possess a greater heating effect than 

 a pound of lead, or iron, or copper at the same temperature. It 

 has been seen by experiment that when equal weights of the same 

 substance are mixed together, the temperature of the mixture is 

 half-way between the temperatures of the two parts which make 

 it. If, however, equal weights of two different substances at 

 different temperatures are mixed together, the resulting tem- 

 perature lies nearer the temperature of the substance with the 



