106 MICRO-ORGANISMS IN RELATION TO MAN. 
It is unnecessary to mention the part they play in the manu- 
facture of beer, wine, and other fermented liquids, but it is 
worth stating in passing that recent experiments prove that 
the distinctive character of a wine depends not so much on 
the grape from which the wine is prepared, as on the variety 
of wine-yeast used in the fermentation. Thus, yeast culti- 
vated from that used in the district of the Garonne, and 
transferred to the juice of the grape in the Burgundy district, 
produces wines of the former class; and yeast from the latter 
district carried to the former takes with it its distinctive 
flavour. 
All putrefaction and decay take place entirely through the 
agency of micro-organisms, and but for them the surface of 
the earth would be covered with the remains of defunct plants 
and animals, undergoing but little more change than the 
inorganic materials of the earth’s crust. Under these circum- 
stances, life as we know it, would soon come to an end, for it 
is by the decomposition of refuse animal and vegetable matter 
in the ground that the fertility of the soil is maintained, and in 
the absence of this decomposition the most fertile land would 
become a barren waste, incapable of supporting plant-life; 
and, with the extinction of the latter, animal life would also 
disappear. 
One of the most important plant-foods in the soil is nitric 
acid in the form of salts; indeed, a soil destitute of this 
substance, is incapable of growing a crop of corn, roots or 
grass, however well-drained, cultivated, and supplied with 
other plant-foods it may be. But in spite of the importance 
of this substance, it is only found in very small quantities in 
any soil at any one time; in fact, the soil under ordinary 
circumstances, continuously generates this nitric acid from the 
various nitrogenous manures applied to it. It was some time 
ago shown that this power of soils to convert the nitrogen of 
nitrogenous substances into nitric acid was due to micro- 
organisms; but the particular organism responsible for this 
