4 BACTERIA, YEASTS, AND MOLDS 



" Know ye not that a little leaven leaveneth the whole lump?" 

 No unportant improvement in this process was made until 

 after the middle of the nineteenth century, when scientists had 

 discovered with their microscopes bacteria and yeasts. Then 

 they found out how to isolate the yeast plant from leaven ; 

 and for the first time were able to teach the housewife 

 something about bread-making. The modem yeast cake is 

 a product of the scientific laboratory ; and its common use 

 to-day is one illustration of the way science can assist the 

 housekeeper in controlling the invisible members of the 

 household. 



The Discovery of Bacteria 



It is not certain who first saw bacteria, but the credit is 

 sometimes given to a Roman Catholic monk, named Kircher, 

 who in 1 67 1 used simple lenses of high magnification to 

 examine various materials such as pus, decaying matter, 

 blood, and water. He mentioned the discovery of "invisible 

 worms" in this material and wondered if they could be the 

 cause of disease. It is very doubtful if he really saw bacteria, 

 for his lenses were of very low magnifying power as com- 

 pared with our modern microscopes ; and his idea that they 

 might have anything to do with disease was a mere guess. 

 Yet it is interesting to know that the existence of micro- 

 scopic organisms was recognized so long ago and that their 

 production of disease had been suspected. 



Only about twelve years later, however, there is even 

 better reason to believe that bacteria were really seen. At 

 this time (1683) a Dutchman, Leeuwenhoek, saw microor- 

 ganisms, described them, and drew pictures of them ; and 

 some of his sketches look very much like actual bacteria. 



