CHAP. xv. COMPARISON WITH OTHER CENTURIES. 153 



steam-engine and the barometer, making seven in all, as 

 against thirteen in our single century. 



Coming now to the theoretical discoveries of our time, 

 which have extended our knowledge or widened our con- 

 ceptions of the universe, we find them to be about equal 

 in number, as follows : 



1. The determination of the mechanical equivalent of 

 heat, leading to the great principle of the Conservation 

 of Energy. 



2. The Molecular theory of gases. 



3. The mode of direct measurement of the Velocity 

 of Light, and the experimental proof of the Earth's Ro- 

 tation. These are put together, because hardly suffi- 

 cient alone. 



4. The discovery of the function of Dust in na- 

 ture. 



5. The theory of definite and multiple proportions in 

 Chemistry. 



6. The nature of Meteors and Comets, leading to the 

 Meteoritic theory of the Universe. 



7. The proof of the Glacial Epoch, its vast extent, 

 and its effects upon the earth's surface. 



8. The proof of the great Antiquity of Man. 



9. The establishment of the theory of Organic Evo- 

 lution. 



10. The Cell theory and the Recapitulation theory in 

 Embryology. 



11. The Germ theory of the Zymotic diseases. 



12. The discovery of the nature and function of the 

 White Blood-corpuscles. 



Turning to the past, in the eighteenth century we may 

 perhaps claim two groups of discoveries: 



