108 
president’s address. 
May, 1889. 
Yeast-Fungi or Saccharomycetes. The various species of 
yeast can be cultivated like the species of Bacteria, in a 
modified form of plate cultivation. But the chief point to 
which your attention is directed is, that the object of the 
process in this case is a purely commercial one, or in other 
words it brings in money. English breweries often suffer 
from fluctuations in the quality and flavour of their beer. 
This arises from the fact that no yeast in ordinary use is pure; 
they all contain more than one kind of Saccharomycetes, 
as well as manv kinds of Bacteria. The latter are reallv 
«' %! 
impurities, which sometimes increase so much in number 
that the yeast ceases to be of any value for the purposes of 
brewing. 
This “degeneration,” as it is called, has long been known 
to practical brewers, but being totally ignorant of the 
microscope, or even of the existence of Bacteria, they 
attributed the cause of it to all sorts of influences but the 
right one. The failure of the beer was assigned to the malt, 
the water, the hops, the temperature of the wort, etc., etc., 
and no two specialists in brewing could ever agree entirely as 
to what it was that caused the degeneration in any given case. 
Still they had invented a practical remedy for this state of 
things, which consisted in the periodic interchange of yeast 
between breweries in different localities. This interchange 
sometimes succeeded in renovating the yeast, and giving it a 
new lease of life. But it was always subject to risks, and 
when one obtained an “ exchange ” it was never possible to 
foresee whether it would be successful. 
The cause of this difficulty obviously lies in the 
impurities which contaminate the yeast. Now by taking a 
small sample of good yeast, and diluting it largely with 
sterilised water, it is found that a tiny drop of the liquid 
taken up on the end of a glass rod, contains only a small 
number of yeast cells. This drop is introduced into a 
sterilised flask containing about 10 cubic centimetres of 
sterilised gelatinous wort—that is, ordinary brewers’ wort 
mixed with gelatine. This gelatinised wort being melted 
and shaken, a small drop is spread evenly upon a microscopic 
cover-glass, and allowed to solidify; it is then placed over a 
moist cell on a microscopic slide, and kept at a temperature 
of 25°C. in an incubator. Each cell entrapped in the 
gelatine is watched beneath the microscope as it develops 
into a colony by budding, and every care is taken that any 
case where two colonies may have coalesced should be 
rejected. We can now select any pure colony, and transfer 
it to another similar flask. By removing the beer that is 
