23 
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, turning 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 gent ration 
was impossible, and Dr. Charlton Bastian on the other, who 
asserted that its possibility had been clearly proved. f 
M. Pasteur claimed that grape juices, brewers’ wort, milk, beet- 
tea, or any other decomposable liquids would remain for an indefi- | 
nite time unaltered if boiled and placed in sterilised tubes, namely,. 
those from which all germ-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 
