202 THE LESSON OF EVOLUTION 



very slow, until, in the seventeenth century, the 

 great improvements which were made in mathe- 

 matical analysis, as* well as the invention of the 

 telescope, enlarged men's ideas enormously, and 

 added vastly to their powers of observation and 

 reasoning. Before the century was over the size 

 of the earth had been ascertained with tolerable 

 accuracy, and the law of universal gravitation had 

 been discovered. In the eighteenth century great 

 progress was made in the experimental sciences of 

 physics and chemistry. Electricity was detected, 

 as also was oxygen ; and this laid the foundation of 

 modern chemistry. Instruments of precision for 

 weighing and measuring were invented ; and, at the 

 end of the century, the distance of the sun was 

 approximately ascertained ; and it was proved that 

 matter was not destroyed when it was burnt, but 

 only rendered invisible. The discovery that matter 

 was indestructible led, in the nineteenth century, to 

 the further discovery that the physical forces are so 

 correlated that one can be changed into another ; 

 and, at last, it was definitely proved that energy 

 was as indestructible as matter, that it was not lost 

 when it was no longer exhibited, but had merely 

 passed into the potential or invisible state. 



Another important result of these investigations 

 was to prove experimentally that matter is inert, 

 and that it exercises no initiative of its own ; that 

 it is moved only by external agencies ; and that , in 

 physics, action and reaction are always exactly equal 

 and opposite ; from which it follows that all material 

 things are under the reign of law. This cannot be 



