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in their optical properties, one being the already-mentioned tartaric 

 acid which rotates the plane of polarisation to the right, the other 

 being a new acid which turns it to the left, thus the optical 

 properties of the one exactly neutralise those of the other, so that 

 the rare acid is optically inactive. M. Pasteur found that the twO' 

 acids thus combined in the rare acid were capable of separation by 

 means of a ferment, the minutest trace of which sets up decom- 

 position in this acid, destroying the common tartaric acid, but 

 leaving unchanged the new and rarer laevorotatory acid. The 

 discovery of this one special ferment, which was the result of 

 seven years' toil, was but the precursor of all those marvellous 

 revelations of the nature of ferments which had previously baffled 

 all chemists, and especially those connected with the processes of 

 wine-making and brewing. These discoveries led eventually to 

 the higher study of bacteriology in connection with infectious 

 diseases to which man and the lower animals are subject. 



Until the year 1857, fermentation was regarded as a somewhat 

 mysterious process, by which a solution of sugar was transformed 

 into alcohol, and alcohol sometimes became vinegar, but that the 

 process of fermentation was due to any specific organism was 

 hitherto quite unsuspected. M. Pasteur, however, succeeded in 

 tracing and identifying distinct and different organisms which cause 

 the several varieties of fermentation. By adding the faintest trace 

 of the acetous ferment to sound wine, he showed that it rapidly 

 developed, turnmg the wine into vinegar. Or, again, milk which 

 was perfectly sweet was in a short time turned sour by the addition 

 of a mere tr ice of the lactic acid ferment. Thus, the individuality 

 of the various germs was proved, for the lactic ferment will not 

 cause acetous fermentation, nor will the alcoholic ferment give rise 

 to lactic fermentation. Then arose the question. How is it that 

 fermentation often appears to arise spontaneously, and that milk 

 turns sour after simply being exposed for some hours to the air ? 

 On this point arose a violent controversy, which raged for many 

 years, the principal exponents of the two theories being M. Pas- 

 teur on the one hand, who maintained that spontaneous gen* ration 

 was impossible, and Dr. Charlton Bastian on the other, who 

 asserted that its possibility had been clearly proved. 



M. Pasteur claimed that grape juices, brewers' wort, milk, beet- 

 tea, or any other decomposable liquids would remahi for an indefi- 

 nite time unaltered if boiled and placed in sterilised tubes, namely,, 

 those from which all gerra-laden air is absolutely excluded. In 

 support of this theory, M. Pasteur placed some beef-tea (a highly 

 decomposable liquid) in a tube with a long narrow neck, in which 

 tube he boiled the liquid ; then by an ingenious contrivance which 

 still kept the neck of the tube red hot, so that all germs were 



