46 CHEMICAL CHANGES. 



woody fibre, gum or starch. It is not more strange 

 than a thousand other things in nature. We have 

 seen, for instance, that carbonic acid puts out all fire 

 and destroys life; yet carbon, one of the substances of 

 which it is composed, burns most violently in oxygen, 

 the other; and this other body, oxygen, is, when alone, 

 the great supporter of vitality : mingled in the air, it 

 is w T hat sustains all animal and vegetable life, and all 

 combustion also. 



It has been incidentally noticed, that certain of the 

 bodies above named may be changed by chemical 

 means. Some of these changes are important, and de- 

 serve a rather more extended notice, a. Woody fibre, 

 if ground fine and subjected to a certain degree of heat 

 for a long time, becomes hard and yellow in color, and 

 finally can be ground like flour. In this state it is 

 partly soluble, and can with yeast be made into a light 

 wholesome bread : it has also been partially changed 

 into a substance resembling starch or gum. b. Starch. 

 if heated at a temperature just below scorching for a 

 day or two, gradually becomes yellow and finally quite 

 soluble, with a sweetish taste. It has become dextrine, 

 or what is called by calico printers British gum. This 

 change takes place to a considerable extent in the 

 ordinary baking of bread, c. By the action of dilute 

 sulphuric acid in certain proportions and at certain 

 temperatures, starch may be changed first into gum, 

 and then into sugar. 



We thus see that this class of bodies are not only 

 similar in composition, but that a change from one to 

 the other may be effected with much ease. If we can 

 do this, how much the more readily can it be effected 

 in the interior of the plant ! That such changes do 

 take place there, and that they are of much practical 

 importance, we shall have occasion to point out in 

 subsequent chapters. 



