A Century of /Science 29 



new world of speculation is opening up before us, 

 full of wondrous charm. The amazing progress 

 made since Priestley's day may be summed up in 

 a single contrast. In 1781 Cavendish ascertained 

 the bare fact that water is made up of oxygen and 

 hydrogen ; within ninety years from that time Sir 

 William Thomson was able to teU us that " if the 

 drop of water were magnified to the size of the 

 earth, the constituent atoms would be larger than 

 peas, but not so large as billiard balls." Such a 

 statement is confessedly provisional, but, allowing 

 for this, the contrast is no less striking. 



Concerning the various and complicated appli- 

 cations of physical science to the arts, by which 

 human life has been so profoundly affected in the 

 present century, a mere catalogue of them would 

 tax our attention to little purpose. As my object 

 in the present sketch is simply to trace the broad 

 outlines of advance in pure science, I pass over 

 these applications, merely observing that the per- 

 petual interaction between theory and practice is 

 such that each new invention is liable to modify 

 the science in which it originated, either by en- 

 countering fresh questions or by suggesting new 

 methods, or in both these ways. The work of 

 men like Pasteur and Koch cannot fail to influence 

 biological theory as much as medical practice. 



