THE INCUBATOR 69 



organism, is added to the media. In milk media, to which litmus 

 tincture has been added (see 85, page 51), organisms pro- 

 ducing alkalies cause the blue colour to be intensified ; others 

 producing acids change it to red ; while others bring about neither of 



FIG. 19. Incubator. 



these changes. Tincture of litmus can also be added to gelatine and 

 agar-agar for the same purpose. 



In ordinary milk cultures coagula also appear, due to acids pro- 

 auced from milk sugar by bacterial action upon the casein of the 



