INTKODUCTOBY. 19 



bastion, except that there is no flame. The air which we 

 breathe into our lungs acts chemically upon the blood, and 

 life is very soon destroyed if this chemistry of the respira- 

 tion be stopped. Chemistry, to a great extent, makes and 

 prepares our food. The grains are made by a union of ele- 

 ments which the plant sucks up from the ground and takes 

 from the air through the pores of its leaves ; and the mak- 

 ing of bread is in part a chemical process, upon the due per- 

 formance of which the goodness of the bread depends. In 

 these examples of chemical action you see the wide range 

 and the practical character of the interesting subjects which 

 chemistry presents to your view. 



13. Changes in the Rocks. In the midst of the chemical 

 changes so extensively and constantly taking place there 

 are some things which are nearly the same from year to 

 year, and even from age to age. The rocks of " the ever- 

 lasting hills" seem to remain unchanged. But it is not so ; 

 there is some change even in them. Heat, air, and water 

 are continually at work upon them, and some portions are 

 thus worn away even from the hardest of them to mingle 

 with the earth. And then, by means of chemical action, 

 these particles from stones and rocks are used in the growth 

 of both plants and animals. The flint that gives strength to 

 the stalks of grain and grass, the lime that is in the shells 

 of eggs and in the bones of animals, and the iron that is in 

 the blood, all came originally from the rocks. 



14. The Sun's Agency. In this chemistry, which is at 

 work so universally, heat is one of the chief agents. And 

 as the sun is the great source of heat, we may think of it 

 not only as giving us light and warmth, but as constantly 

 stimulating to the changes which are taking place among 

 the elements that are within and around us. Not only so, 

 but, as you will see in the course of our investigations, there 

 is a special chemical force bound up with the light and heat 



