44 ELEMENTS OF LABORATORY WORK 



altered from C. to the observed temperature of the mix- 

 ture. These two changes have proceeded simultaneously 

 with the temperature-change in the quantity of water first 

 taken, which has fallen from its original temperature to that 

 of the mixture. By making use of the definition of the unit 

 temperature-change we can obtain a numerical value for the 

 latter temperature -change, and likewise one for the water 

 yielded by the ice in rising from to the final temperature. 

 The difference between the two magnitudes gives the numeri- 

 cal value corresponding with the change of state of the quan- 

 tity of ice which has been used ; and from this it is easy to 

 calculate the numerical value for 1 gram of ice. 



The most accurate values obtained have been from 79 to 

 80, but this experiment will only give approximate results, 

 since the changes produced in the air and the beaker itself have 

 not been taken into account. Greater accuracy may, how- 

 ever, be obtained by enclosing the beaker in non-conducting 

 material, such as cotton-wool, and finding out how much 

 water the glass of the beaker would be equivalent to, and 

 adding this to the other quantity thermally changed. 



We must infer that the melting of ice is always accom- 

 panied by changes equivalent to those here measured, what- 

 ever the surrounding bodies may be. 



It is convenient in thermal measurement to consider the 

 unit change of temperature as caused by a unit quantity of 

 heat. We may then shortly describe the above process by 

 saying that 80 units of heat are required to melt 1 gram of 

 ice. If the temperature of a body rises, it is said to gain 

 heat ; if it falls, to lose it. But as we are able to observe 

 directly only such changes in the properties of bodies as we 

 have agreed to class together as temperature-changes, or as 

 caused by temperature-changes, it is important to remember 

 that the terms, gain, or loss of ' heat,' are merely convenient 

 expressions for changes of temperature. More exact know- 

 ledge of temperature-change cannot be acquired until its co- 

 relation with other physical changes is shown. We shall 

 then learn what it is that is gained or lost during change of 

 temperature, and why it is that the passage of ice, and of all 



