Heat as a Quantity 171 



In this experiment the result obtained was ex- 

 tremely close to that generally given as the most probably 

 correct value, but this is not always the case with 

 experiments done as described, since small errors are 

 practically unavoidable, and, further, the specific heat 

 of the glass is only an average value, and that of the 

 glass actually used may be different to the extent of 

 over 10 %. With care values for the latent heat may 

 be found which will lie between 75 and 80. 



EXPERIMENT 58. Arrange the apparatus illustrated 

 in Fig. 52. A is a screen of wood or asbestos to prevent 

 direct heat from the flame and the heated flask from 

 reaching the beaker and its contents. T is a " steam- 

 trap". Its function is to prevent as far as possible 

 the entrance of hot water, formed by the condensation 

 of a portion of the steam, into the water contained 

 in the inner beaker. Weigh this beaker empty. Half 

 fill it with water in which sufficient ice has been melted 

 to bring it to a temperature of about 5. Weigh the 

 beaker and cold water, and replace it in the larger 

 beaker. As in the last experiment, the space between 

 the beakers is packed with cotton- wool. Heat the 

 water in the flask to boiling-point, and, after steam 

 has been allowed to escape from the lower end of the 

 trap for about two minutes, put the lower end of the 

 trap in the water in the beaker, noting the exact 

 temperature of the water at the instant of introducing 

 the tube. Let the steam continue to pass through the 

 cold water till the temperature of the latter has risen 

 about 20. Remove the end of the trap, stir the water 

 well with the thermometer, and record the highest 

 temperature registered. Weigh the beaker and con- 

 tained water. 



The latent heat of vaporisation of water was 



