﻿32 THE AIR OF TOWNS. 



in the malt, and to this its sweetness is due. The malt is now steeped 

 in water for a short time, the water is boiled and rapidly cooled, and 

 this extract is called "wort." If a little yeast or brewer's barm is 

 added to the wort it begins shortly to bubble up, and at the same time 

 a white scum forms. This scum is yeast, which by the end of the proc- 

 ess is foar or five fold the quantity of the original yeast. The sweet- 

 ness of the wort has gone, and in the place of sugar it now contains 

 alcohol. The making of wine and the brewing of beer are very simi- 

 lar processes. In brewing, the brewer adds his ferment j in wine mak- 

 ing, the ferment is with the grape. ''What has been done consciously 

 by the brewer has been done unconsciously by the wine grower."^ The 

 nature of this ferment — the yeast — was first examined in 1680 by 

 Leuwenhoek in the early days of the microscope, and he found that it 

 consisted of minute globules. More than a century and a half elapsed 

 before our knowledge of these globules was materially increased, and 

 then in 1835 Cagniard de la Tour, in France, and Schwann, independ- 

 ently, in Germany, on carefully observing these globules noticed that 

 they threw out buds, that they were in fact a low form of plant life. 



Here you see (fig. 30) the yeast plant in its various stages of growth, 

 the single spherical cell, then the bud growing and developing, and 

 finally separating from the mother cell and so forming a new yeast plant. 

 If the liquid is undisturbed these cells remain together and appear to 

 ramify like the lobes of a cactus leaf. 



It was at this point that Pasteur took up the subject. I could easily 

 devote a lecture — nay, a series of lectures — to the researches of this 

 distinguished chemist, which are models of scientific acumen and 

 experimental skill. 



It may suffice to say that he incontestably established the fact, in 

 spite of much opposition on the part of scientific men, that the conver- 

 sion of sugar into alcohol is brought about, although we do not yet 

 know how, bj^ the living yeast cell during its life in the liquid. As 

 long as yeast is excluded no fermentation takes place. How comes it, 

 then, that wine ferments spontaneously, whereas beer does not? This 

 question was also answered by Pasteur. The germs of the yeast plant 

 are contained in the dust of the air which settles upon the grape. I 

 will now show you on the screen the apparatus and explain the method 

 by which Pasteur solved the problem. 



The flask A (fig. 31) has two necks, the one is drawn out to a point, 

 and sealed, the other is also drawn out to a fine tube and bent, as 

 shown in the figure. Although the end is turned up and open, no dust 

 can enter. The point is inserted through the skin of the gTape, as 

 shown in B in the enlarged drawing of the same. After insertion the 

 point is broken and the juice sucked into the flask by aspirating at the 

 open bent limb. The point was then fused 5 in this way the dust from 

 the outside of the grape was excluded and no fermentation took place. 



^ Professor Tyndall. 



