OBSERVATIONS OF CHANGE OF TEMPERATURE 41 



thermometer showing its temperature. Convey the copper to 

 the beaker as quickly as possible by means of the silk, and 

 note the change of temperature taking place. 



The temperature of the known quantity of water rises, 

 whilst that of the known quantity of copper falls, until there 

 is thermal equilibrium ; but the resultant temperature shows 

 that the fall is not equal to the rise, nor are the respective 

 changes proportional to the relative quantities of copper and 

 water present. 



Although it leads to greater accuracy to use a relatively 

 large quantity of water in this experiment, we may infer 

 that with equal quantities of water and copper different tem- 

 perature-changes would be reciprocal. Observations with 

 other substances indicate, similarly, that the same temperature 

 changes (as marked by the thermometer) are not equivalent 

 for different kinds of matter. It must be remembered that 

 only approximate results will be obtained unless we prevent 

 surrounding bodies, including the air, from taking part in the 

 thermal changes without being estimated in our calculations. 

 Calorimeters, such as already described, are intended to effect 

 this requirement. 



A unit of temperature- change is required for purposes of 

 measurement. Pure water at any temperature between 

 and 4 is selected as the standard substance in which it is to 

 be observed. 



A change of 1 C. in 1 gram of water at any tempera- 

 ture between and 4 C. constitutes the unit temperature- 

 change, and forms a basis for thermal measurements. If 

 2 grams of water are thus altered there are 2 units of tem- 

 perature-change, and if 2 grams alter by 2, as indicated by 

 the thermometer, the numerical value of the total temperature- 

 change is 4. 



Accurate observation shows that very nearly the same 

 numerical values are obtained if the temperature- changes, 

 which commence on a higher point of the scale, are taken as 

 equal to similar changes between and 4 C. In rough ex- 

 periments this may always be done. A change from 20 to 

 21 may be considered equal to a change from 2 to 3, 



