ACID FERMENTATION 151 



growth whicli had the power of exciting a vigorous fermenta- 

 tion in suitable solutions of calcium lactate. The composition 

 of the medium was as follows : — 



Calcium lactate, 3 grams ] , , „^„ 



■o . 1 . • on made up to dOO c.c. with 



Peptone solution, dO c.c. [ t ■,, ^ 



r, 1 ■ I. i. o distilled water. 



Calcium carbonate, 6 grams ) 



This solution was inoculated with a minute quantity of 

 calcium lactate solution in active fermentation. 



The quantity of nutrient solution, its concentration (3 per 

 cent, instead of 1 per cent., as above, being occasionally 

 used), and the duration of the fermentation were varied in 

 different cases. At the end of each experiment the calcium 

 carbonate was filtered off and the filtrate concentrated and 

 examined in the polarimeter. The calcium was removed from 

 solution by means of oxalic acid, and the filtrate from the 

 calcium oxalate evaporated on a water-bath to remove volatile 

 acids. The lactic acid remaining was separated from other 

 impurities by precipitation with lead acetate, decomposition of 

 the lead salt with HgS, evaporation of the filtrate from the lead 

 sulphide, and extraction with ether. The residue after eva- 

 poration of the ether was converted into the zinc salt by boiling 

 with zinc carbonate, and the solution of the zinc salt was again 

 examined in the polarimeter. This zinc salt was found to be 

 pure Isevo-rotatory zinc lactate. The calcium lactate originally 

 taken was inactive, so that there had evidently been prefer- 

 ential decomposition of the dextro salt. If the fermentation 

 was stopped at too early a stage, the active lactate was found 

 to be mixed with a large quantity of inactive lactate, whilst 

 when the fermentation was too long continued, the active, 

 lactate was also destroyed. The above description will serve 

 to illustrate the kind of investigation necessary for determining 

 the precise products of a reaction, when there is a possibility 

 of one or another stereo - chemical modification being pro- 

 duced. The production of an inactive or active modification 



