168 Domestic Science 



If the water be at about the average temperature 

 of water from the tap, say 15, before the heating is 

 started, it will be found that it requires about six times 

 as long to turn the water at 100 into steam as it does 

 to raise its temperature from 15 to 100. Sinpe 

 20 x (100 - 15) = 1700 calories are needed to effect the 

 latter change, it follows that some 10,200 calories are 

 used up in merely changing the physical state of the 

 water. For every gram of water over 500 calories 

 are rendered " latent " during the vaporisation. A 

 more exact value is 536. 



Note. It is possible, of course, that the steam is 

 hotter than the boiling water, and that some of the 

 heat is used in raising the temperature of the steam. 

 To test this, repeat Experiment 48, and note whether 

 the temperature of the steam evolved from the boiling 

 water shows any increase of temperature during 10 

 minutes' boiling. 



Other substances possess latent heats of fusion and 

 vaporisation, and some examples are given in the 

 subjoined table : 



Substance L. FT. of Fusion L. H. of Vaporisation 



Water.. 



Mercury 



Nitrate of soda 



Sulphur 



Alcohol 



Ether.. 



Turpentine 



80 536 



2-82 62 



63 



9-4 



202 

 90 



74 



109. To determine the latent heats of fusion and 

 vaporisation of water with a greater approach to 

 accuracy than was possible in the experiments already 

 described, the following experiments should next be 

 performed. 



