THE USE OF BACTERIA IN THE ARTS. 57 



its formation by bacteria was historically one of 

 the first bacterial fermentations to be clearly un- 

 derstood. It can be produced also in various 

 sugar and starchy solutions. Glycerine may also 

 undergo a butyric fermentation. The presence 

 of this acid is occasionally troublesome, since it 

 is one of the factors in the rancidity of butter and 

 other similar materials. 



INDIGO PREPARATION. 



The preparation of indigo from the indigo plant 

 is a fermentative process brought about by a spe- 

 cific bacterium. The leaves of the plant are im- 

 mersed in water in a large vat, and a rapid fer- 

 mentation arises. As a result of the fermentation 

 the part of the plant which is the basis of the in- 

 digo is separated from the leaves and dissolved in 

 the water ; and as a second feature of the fer- 

 mentation the soluble material is changed in its 

 chemical nature into indigo proper. As this 

 change occurs the characteristic blue colour is de- 

 veloped, and the material is rendered insoluble in 

 water. It therefore makes its appearance as a 

 blue mass separated from the water, and is then 

 removed as indigo. 



Of the nature of the process we as yet know 

 very little. That it is a fermentation is certain, 

 and it has been proved that it is produced by a 

 definite species of bacterium which occurs on the 

 indigo leaves. If the sterilized leaves are placed 

 in sterile water no fermentation occurs and no 

 indigo is formed. If, however, some of the spe- 

 cific bacteria are added to the mass the fermenta- 

 tion soon begins and the blue colour of the indigo 

 makes its appearance. It is plain, therefore, that 



