SCIENCE OF COMMON THINGS. 303 



Composition of wheat flour. Acids. Alkalies. 



perature, which causes the cells to swell, thereby giv- 

 ing to the vegetable a rounded appearance. 



1947* What is the composition of wheat flour? 



Starch is one of the principal constituents of wheat 

 flour, as well as of all other kinds of meal. The other 

 principal constituent is a grey, tough, viscous sub- 

 stance, called gluten. 



1048 To what does paste, made of wheat or rye flour, owe its adhe~ 

 siveness f 



In some measure to the starch, but principally to the 

 gluten contained in it. 



1949 Can starch be converted into gum. and sugar f 



It can ; fruits and plants effect this change natu- 

 rally : we can also produce the change artificially by 

 chemical * rocesses. 



50 SO Why are potatoes frozen and thawed sweet f 



Because by the freezing action the starch of the po- 

 tato is in part converted into sugar. 



1951 Why are apples, pears, grapes, etc., in their unripe state sour, 

 and in their ripe condition sweet ? 



In the unripe fruits mentioned starch is present ; in 

 the ripe fruits it is absent in the process of ripening 

 the starch is converted into sugar, and the fruit losing 

 its sour taste, becomes sweet. 



1953 What are acids f 



Acids are substances which excite the taste of sour- 

 ness when applied to the tongue ; they change the blue 

 juices of vegetables to red, and combine with alkalies 

 to form neutral compounds. 



1953 What is an alkali? 



An alkali is a body that possesses properties the 

 converse of those of an acid. It has a highly bitter, 

 acrid taste, changes the blue juices of vegetables green, 

 or the juices of vegetables which have been changed 

 red by an acid, back again to blue. Potash and soda 

 are the representatives of the alkalies. 



3.954 When sulphur is burned in the air what is the product formed 1 



Sulphurous acid. 



