COMPOSITION OF LIFELESS AND LIVING THINGS 21 



2. (Optional.) Test as many foods as you can by warming in turn 



a small quantity of each in a spoon (without letting the food 

 burn), and holding over the spoon a dry, cool tumbler. 

 What do you learn as to the presence of water in foods ? 



3. (Optional Demonstration.) To determine the amount of water 



in potatoes: 



a. Remove a thin layer of peel from a potato, weigh the potato, 

 and lay it aside in a warm dry place (protected from mice). 

 Weigh each day, and fill out in your note-book for each 

 day the first, third, and fifth columns in a table like the 

 following : 



b. Secure a second potato about the same size as the first, weigh 



it each day, and place it beside the potato that was peeled. 

 Record results and percentages for each day in second, 

 fourth, and sixth columns of preceding table. 



c. Which of the two potatoes decreases in weight the more 



rapidly ? Almost all the loss of weight is due to the evapo- 

 ration of water; what do you infer, therefore, as to one 

 use of the peel ? 



d. Continue to weigh the potato without the peel at intervals 



until there is no further loss. What percentage of potato 

 is water? When potatoes are bought at $3 per barrel, 

 how much of this sum is paid for water ? 



