292 EIGHTEENTH CENTURY. PT. in. 



till they have traced it home to its first and simplest elements. 

 We need not recapitulate here the different discoveries of 

 Scheele, Bergmann, Black, Cavendish, Priestley, and La- 

 voisier. You will remember how they all helped to overthrow 

 the theory of Phlogiston, and to prove that combustion and 

 respiration are merely chemical changes taking place between 

 different substances and the oxygen of our atmosphere ; and 

 this truth is the starting-point of modern chemistry. 



Physics. In physics the three subjects in which we 

 have noticed advances are those of sound, heat, and elec- 

 tricity. In the first of these the interest is chiefly theoreti 

 cal, and we learn how the melodies and harmonies which 

 are so pleasing to our ears are the result of vibrations act- 

 ing under fixed laws, so that we can calculate with the 

 greatest exactness what are the movements of any particular 

 body giving out a musical sound, and can even convert the 

 sound, as it were, into a visible figure in which we can read 

 the history of the note's production. The discoveries in the 

 other two subjects, heat and electricity, are of great practical 

 interest, and have caused greater changes throughout the 

 world than any previous discoveries. When Black proved 

 the amount of heat which is lying hid in water and in 

 steam, and when Watt applied it to the steam-engine, a 

 giant power was born into the world which has worked 

 marvels. Visit any of the little towns all over England and 

 see the machines of all kinds moved by the simple power of 

 steam : then go to the great manufacturing towns and see 

 the huge engines doing the work of thousands of horses, 

 with no other help than that of a man feeding the furnace 

 with coals. Look at any one of your own clothes, at the iron- 

 work in all parts of your house, from the rough heavy iron 

 of fireplaces and fenders to the delicate steel spring which 

 moves the hands of your watch ; look at the planks on youi 



