52 
Pasteur and his Work, 
to abandon his hitherto congenial and highly successful line of 
research in the domain of chemistry and molecular physics, and 
enter upon a new but not very dissimilar course, in which his 
great natural gifts and previous training were to confer such 
advantages. This was the very important study of fermenta- 
tion, to which his mind was attracted by an almost casual 
incident while he was at Strasburg. 
The observations of a manufacturer of chemicals in Germany 
had long made it known that the impure tartrate of lime of 
commerce, if contaminated with organic matters, and allowed 
to remain dissolved in water during warm weather, fermented, 
and yielded various products. This excited Pasteur's curiosity, 
and he prepared some pure right-handed tartrate of ammonia,* 
to which he added some albuminous matter, and placed the 
liquid in a warm chamber, where it fermented. During the 
process of fermentation, the previously limpid mixture gradually 
became turbid, and the turbidity was found to be due to the 
presence and multiplication of a microscopic fungus, which, 
obtaining its sustenance in the liquid, acted as a living ferment. 
To the paratartrate of ammonia this mode of fermentation 
was also applied successfully, the same organism appearing, 
though there was a wide difference between the results of the 
two fermentations, so far as the products were concerned ; and 
the important fact was established, that the molecular dissym- 
metry proper to organic matters intervened in a phenomenon 
of the physiological order, and did so as a modifier of chemical 
affinity. The little fungus was able to assimilate the right- 
handed tartrate more readily than the left, though there was no 
chemical difference between them — only a difference in mole- 
cular constitution. Pasteur was, in this way, the first to introduce 
into physiological consideration the fact of the influence of the 
molecular dissymmetry of natural organic products, and in de- 
monstrating that the common mould or mildew could live and 
multiply on a purely mineral soil, such as the phosphates of 
potash, of magnesia, and an ammoniacal salt of an organic 
acid. Sowing the seeds of this mould — Penicillium glaucum — in 
a solution of pure paratartrate of ammonia, it was seen that in 
germination the left-handed acid appeared in proportion as the 
right-handed disappeared, the only aliment the plant obtained 
for its growth being the carbon in the tartaric acid. 
These remarkable experiments led Pasteur to infer that fer- 
ments were always living organisms, what had previously been 
looked upon as ferments being merely their food. The yeast- 
plant had been previously discovered by Leuwenhoeck in Hol- 
* A salt which turns the plane of polarised light to the right ; there is also a 
left-haiuled tartrate of ammonia. 
