^ APPLIED BACTERIOLOGY 



they grow, or acquire a colour of their own. If some 

 slices of boiled vegetables, such as carrots or potatoes, be 

 exposed to the air for a few minutes, and then covered up 

 and allowed to remain for a few days in a warm place,, 

 bacterial colonies will be seen to have developed, spreading 

 over the surface of the media, giving rise to characteristic 

 appearances, such as various white and coloured patches in 

 the form of little droplets, or more or less slimy masses 

 with irregular outlines. While the slices of vegetables 

 were exposed to the air, various bacterial germs fell upon 

 them, and then developed at the spots where they fell 

 into colonies, which remain isolated on the solid media. 

 Occasionally, however, it may happen that two or more 

 germs fall on the same spot, in which case the resulting 

 colony is impure from the first. 



The history of the science of bacteriology may be said to 

 commence with the observations of Anton Leuwenhoeck, of 

 Delft, Holland, who in 1675 constructed a microscope of 

 sufficient power to demonstrate minute organisms in water, 

 putrefying fluids, saliva, etc., of a kind which up to that 

 time were quite unknown. 



A century later, namely, in 1775, the Danish investigator 

 MuUer named and described some three hundred organisms 

 occurring in the waters about Copenhagen. The first .ex- 

 periments in connection with the sterilisation of apparatus 

 by heat were made by the Abbe Spallanzani, about the 

 year 1775. 



Scarcely any advance was made in our knowledge of the 

 bacteria until Ehrenberg, the naturalist, in 1830 studied 

 them with the aid of improved instruments. The lack of 

 culture-methods, however, prevented him from recognising 

 the true nature of these organisms. 



Cohn a few years later shed fresh light upon the subject 

 by showing that the bacteria are plant-cells, with which 



