Latent Heat. 35 



and when the walls of the creamer are made of so poor a 

 conductor of heat as to admit as little as possible from 

 without. 



48. Washing with Snow or Ice. When ice or snow arc 

 used in winter for washing purposes there is a large loss 

 of heat incurred in simply melting the ice and raising the 

 temperature of the water from 32 F. up to 45 F., the 

 temperature it may have in any well protected cistern. 

 To melt a pound of ice and raise its temperature to 45 F. 

 will require 



142 + 13 = 155 heat units, 



If 300 pounds of water are required for a washing then 

 the lost heat will be 



300 X 155 = 46,500 heat units. 



The fuel value of one pound of water-free, non-resinous 

 wood, such as oak or maple, has been found to be 15,873 

 heat units ; that of ordinary dry wood, not sheltered, con- 

 taining 20 per cent, of water, is 12,272 heat units. At 

 this latter value it will require, supposing 50 per cent, of 

 the fuel value to be utilized in melting the ice and heating 

 the water, 



more than would be needed to do the same washing with 

 water at 45 F., to say nothing of the expense of getting 

 the snow or ice and the unhealthf ulness of handling it. 



48a. Burning Green or Wet Wood. Whatever water 

 wood or other fuel may contain when it is placed in the 

 stove, so much of the fuel as is required to evaporate this 

 water must be so expended and is prevented from doing 

 work outside of the stove. We have seen (48) that when 

 wood contains 20 per cent, of water there is required 



15,873 12,272 = 3,601 heat units 



per pound of wood to evaporate the water contained, which 

 is 22.7 per cent, of the total value. Wood, after being in 



