96 Fermentation and Kindred Phenomena. 



saccharine medium, he saw them multiply and produce all the 

 effects of the lactic fermentation. As milk does not sour if 

 taken from the cow in such a manner that no dust or solid 

 particles can fall into it, there can be no question that its souring 

 is due to the introduction of the spores of the ferment from 

 dust or air. 



The Butyric Ferment. — In a sugar solution which is under- 

 going lactic fermentation there is commonly developed, especially 

 towards the close of the operation, another acid, which from its 

 occurrence in rancid butter is called butyric acid. Pasteur 

 investigated the causes of its production, and found that here 

 again a minute organized ferment was at work, causing sugar 

 to undergo a perfectly definite decomposition into butyric and 

 carbonic acids and hydrogen gas. The change may be repre- 

 sented by the following equation : — 



C 6 H 12 O e = C 4 H 8 O a + 2C0 2 + H 2 



Sugar. Butyric Acid. Carbonic Acid. Hydrogen. 



This change is remarkable on account of the hydrogen which is 

 produced, for I do not think there is any other instance known 

 in which it is formed under the influence of a living vegetable 

 organism. Advantage is taken of the circumstance in dyeing 

 wool and cloth with indigo, the dyer employing a vat containing 

 indigo diffused in water and a coarse kind of wheaten flour or 

 bran. The starch which the latter contains is first transformed 

 into sugar, which is eventually decomposed by the butyric 

 ferment, and the hydrogen which is liberated converts the indigo 

 into a colourless soluble substance which is readily absorbed by 

 the wool, but which is again converted into the indigo and 

 precipitated within the fibre when the wool is exposed to air. 

 I may mention that sugar is not the only substance upon 

 which the ferment acts, for it will also decompose tartaric, mucic, 

 and malic acids, and convert them into butyric acid. 



" The butyric ferment resembles the lactic ferment in appear- 

 ance, consisting of rods or bacilli. 



The fact that sugar is capable of fermenting in three different 

 ways, and that these fermentations occur spontaneously, leads 



