OBSERVATIONS OF CHANGE OF TEMPERATURE 



45 



other solids, to the liquid state involves a thermal change in 

 surrounding bodies without any change in their own tempera- 

 ture, 



35. To Measure the Temperature-change in a Known 

 Quantity of Water corresponding with the Gasification of a 

 Known Quantity of Water, or the Numerical Value of the 

 Temperature-change involved in changing 1 gram of 

 Water at 100 into Steam at 100. A known quantity of 

 water is taken in a glass beaker, its temperature is ascertained, 

 and then steam or water-gas is allowed to pass into it for a 

 short time. The quantity of steam added and the change of 

 temperature produced is easily deter- 

 mined, but it is important that only 

 water gas and not condensed steam be 

 added, or the determinations will be 

 inaccurate. With this purpose the 

 tube A, leading the steam into the 

 water, is a narrow tube, fitted with a 

 cork into the bigger tube B, so that 

 the steam leading into the water is 

 maintained at 100, and is shielded 

 from the cooler air by the steam in B, 

 which will be found to condense partly. 

 The water in the beaker c should be 

 shielded from the effects of the flame 

 by which the water in the flask D is 

 made to boil. 



This method is not direct, and we have to assume that the 

 change is reversible. That is, the gasification of a given 

 quantity is accompanied by the same temperature-change as 

 when it is condensed, except that the direction of the change 

 is reversed. In the first case the temperature would be 

 lowered (as it was in the melting of ice), in the latter it is 

 raised. The truth of this is proved by many observations. 



As in the case of the liquefaction of ice, there are two 

 operations together balancing a third. The rise in tempera- 

 ture of the known quantity of water co-exists with the con- 

 densation of a known quantity of steam, together with the 



Fig. 14. 



