356 
POPULAR SCIENCE REYIEW. 
denotes tbe number of atoms in the compound ; the different 
symbols written side by side denote the compound made up by 
the different elementary atoms. 
A definite quantity of the fungus, or yeast, as it is called 
generally, is, however, capable of converting a definite quantity 
only of sugar into carbonic acid and alcohol; after which the 
fungus loses its vitality and perishes. 1^ parts of dry yeast are 
capable of decomposing about 100 parts of sugar. Hence, if in 
a saccharine solution there is not a sufficient amount of nitro- 
genous matter present for the growth and nutrition of the 
necessary quantity of yeast, some of the sugar will remain 
unaltered ; if, on the other hand, there is more nitrogenous 
matter than is necessary, part of this latter will remain un- 
changed. 
Alcohol and carbonic acid are not, however, the only products 
of the fermentation of sugar ; but there are produced simul- 
taneously and invariably a certain proportion of glycerine 
and succinic acid, and probably also a variable proportion of 
some of the higher homologous * alcohols, as propylic, butylic, 
amylic, &c. &c. 
The yeast plant, moreover, though the only substance inducing 
alcoholic fermentation in saccharine solutions, is not the only 
ferment capable of decomposing sugar. Under the influence 
of putrefying casein, for example, sugar yields lactic and butyric 
acids ; and it is highly probable that a small part of the sugar in 
the must suffers a similar decomposition under the influence of 
the vegetable casein and analogous substances present in small 
quantities in the grape juice. The breaking up of the sugar 
into carbonic acid and alcohol, &c. &c., is, however, by no means 
the only chemical action going on in the fermenting must. 
During the first few days of the fermentation, when the 
evolution of carbonic acid is copious, atmospheric air is no 
doubt almost entirely cut off from the must^ by the layer of 
carbonic acid gas constantly covering the surface. As soon, 
however, as fermentation becomes less energetic, atmospheric 
* Among carbon compounds there are many that may be arranged in 
series, the successive members of which differ by an increment of CHg, as 
illustrated by the series of alcohols and series of fatty acids : — 
ethylic alchohol CgHg 0 
propylic „ CgHg O 
butylic „ C^HjgO 
amylic „ CgHjgO 
acetic acid 
propionic „ CgHgOg 
butyric „ C^HgOj 
valeric „ 
The members of all such series, termed homologous series, bear a great 
resemblance to each other in general chemical characters, such resemblance 
being the greatest between those standing nearest to each other in the series, 
and getting less and less as their difference in composition increases. 
