6 i Phenomena of Fermentation^ [Book VIII, 



boiling immediately on being expreiTed. Preierves, 

 and other mixtures prone to fermentation, are pre- 

 vented from that procefs by the fame method. 3. A 

 proper temperature, which varies from forty-eight to 

 eighty of Fahrenheit's thermometer. If below this, 

 the fermentation is languid j if above it, it is impetu- 

 ous, and is apt to rum into the acetous ft ate even be- 

 fore the vinous. 4. The addition of a quantity of the 

 fubftance called yeaft, which is itfelf the product of the 

 vinous fermentation^ is of great afiiftance in exciting it. 

 By what power yeaft acts in producing the vinous fer- 

 mentation, has been much difputed. Mr. Henry 

 thinks that yeaft is ne other than fixed air already 

 formed, but enveloped or entangled in the mucila- 

 ginous matter of the liquor from which it was obtained ; 

 and the fame ingenious experimentalift was able to 

 bring on the vinous fermentation* by adding to com- 

 mon wort a quantity of fixed air in the elaftic form. 

 To account for this, it is not difficult to fuppofe that 

 fixed air has an attraction for its own conftituent prin- 

 ciples, when placed in favourable circumftances to act 

 upon them ; and that it will thus occafion the fepara- 

 tion of fixed air from the fermentable liquor, which 

 is fo remarkable during fermentation. 



The phenomena which prefent themfelves in a li- 

 quor during the fpirituous fermentation are Firft, 

 A muddinefs, from the ftparation of an aerial matter, 

 which rifes in bubbles to the top in fuch quantity, 

 and in fuch quick fucceffion, as to produce a hifling 

 noife, and form a froth. Thefe minute globules of 

 air occafion the motion of the particles of the fluid 

 among one another ; and this motion is perceptible, 

 even before the air is vifibly feparated. The globules 

 of air attach themfelves to the particles of the mix- 

 ture, and buoy them up; at length the globule is 



detached, 



