100 BACTERIOLOGY. 



rotatory lactic acid were known. In this con- 

 nection it is also worthy of note that bacteria, 

 yeasts and moulds exert a selective power in 

 their action upon inactive bodies. If either 

 yeast or the common blue-green mould (Pen- 

 idllium glaucum) is fed upon racemic acid, that 

 is, upon a mixture of right and left-handed tar- 

 taric acid, the right-handed tartaric acid is used 

 up and the left-handed remains behind ; the 

 same is true with methylpropylcarbinol and 

 leucin. If the mould acts upon inactive amyl- 

 alcohol or amygdalic acid, the left-handed modi- 

 fication is consumed and the right-handed is 

 left untouched, while, if dealt with separately, 

 the latter, which is not attacked in the mixture 

 or inactive compound, can, when alone, serve as 

 nutriment to the mould and be consumed by it. 

 The laevo-rotatory fruit-sugar as well as the 

 dextro-rotatory grape-sugar is broken up by 

 yeast in the alcoholic fermentation ; in a mix- 

 ture of both, such as occurs in the invert-sugar 

 -of honey, the grape-sugar is fermented and 

 the left-handed fruit-sugar remains unfer- 

 mented. The same thing happens in a mix- 

 ture of the right-handed galactose derived from 

 milk-sugar with fruit-sugar, and this, in spite 

 of the fact that when by itself, fruit-sugar 

 is attacked by most yeasts more readily than 

 galactose. The different geometric arrange- 



