136 PERIOD V. 



and elevated, and how ethical motives at length became 

 influential if not predominant. 



Pasteur's Experimental Study of Microbes. 



The same difficulty arises with Pasteur as with 

 Darwin ; his life-work has already been described often 

 and well. Readers unversed in science have only to 

 turn to the Vie de Pasteur, written by his son-in-law, 

 Vallery-Radot, to find a luminous account, giving- just 

 so much detail as makes the discoveries intelligible and 

 interesting. If shorter sketches are demanded, they 

 exist. We must therefore above all things be brief, 

 and content ourselves with reminding the reader of 

 facts which, in spite of their recent date, are as well 

 known as anything in the history of science. 



Chemists will claim Pasteur as one of their number, 

 and we do not dispute the claim. Trained in experi- 

 mental methods by the chemical laboratory, he devoted 

 his best powers to the study of living things, and, with- 

 out ceasing to be a chemist, became one of the greatest 

 of biologists. 



Pasteur's chief work was of course the experimental 

 investigation of living particles which float in the air 

 what we may call live dust. Before his day such 

 particles had been seen, named, and classified ; some 

 few had been studied in their action and effects. Most 

 of them are plants of low grade, simplified to the last 

 point for the sake of minuteness, on which their ready 

 dispersal depends. 



Yeast. --Van Helmont, early in the seventeenth 

 century, when the microscope had not yet become an 

 instrument of research, attempted to investigate the 

 fermentation of beer, and made acquaintance with the 

 properties of the gas which is evolved, his gas stlvestre > 



