FORMATION OF YEAST. 177 



is termed foxlness. can never be completely remedied 

 if it has once commenced. It is peculiarly common 

 in moist, close, warm weather. 



423. The cooled wort is then run into the ferment- 

 ing vat or gyle-tun ; if left to itself, fermentation 

 would soon commence ; but it is far better to add at 

 once some good yeast from the last brewing, stir the 

 whole up well together, and then, having covered the 

 vat, leave it quiet ; in a few hours the whole will be 

 in an active state of fermentation. 



424. In three or four hours, bubbles of gas will be 

 seen rising from all parts of the liquid, a ring of froth 

 forming at first round its edge, and gradually increas- 

 ing and spreading till it meets in the centre, and the 

 whole surface becomes covered with a white creamy 

 foam. The bubbles of gas then rise and break in 

 such numbers, that they emit a low hissing sound, 

 and the white foam of yeast continues to increase in 

 thickness, breaking into little pointed heaps, which 

 become brownish on the surface and edges. 



425. The yeast gradually thickens, and at last 

 forms a tough, viscid crust, which, when the ferment- 

 ation begins to slacken, would break and fall back 

 again into the vat, if it was not removed. The 

 brewer skims it off and removes it as soon as he 

 judges that the fermentation is complete; but if he 

 thinks that the fermentation has ceased too soon, he 

 stirs the whole up again so as to mix the yeast with 

 the wort. This, which is called rousing the vat, pro- 



