THE AIR OF TOWNS. 379 



Yeast would also find its way into the brewer's wort; but this liquid is 

 neutral and not acid like grape juice and is capable of nourishing other 

 germs, which can not convert sugar into alcohol, but yield acid sub- 

 stances, as the brewer not unfrequently finds to his cost, when occa- 

 sionally such germs find their way into the fermenting vat. 



By adding pure yeast, the yeast being first in the field establishes 

 itself generally to the exclusion of other forms of life just as soil sown 

 with wheat will produce wheat and not weeds, as it would otherwise 

 do. The souring of beer and wine next claimed Pasteur's attention 

 and he found that certain much more minute forms of low vegetable life 

 called bacteria or microbes had the property of converting sugar into 

 acids. 



Here are some of these much more minute germs which are found in 

 bad beer growing in bead-like filaments side by side with the yeast 

 cells (fig. 32). The study of the microbes led Pasteur to the discovery 

 of a process for preventing wine from turning sour. He found that a 

 temperature below the boiliug point of water destroyed these germs. 

 After the wine is bottled a short immersion in hot water will kill the 

 germs without materially affecting the flavor of the wine, and the wine 

 will undergo no change on keeping. This process is known as " pasteur- 

 ization." The production of vinegar from beer and wine was found to 

 be due to the microscopic ferment, which converts alcohol into acetic 

 acid, known as mycoderma aceti or acetic ferment, and which, as just 

 stated, is found in sour beer. 



The germs of all these forms of vegetable life are found in the dust of 

 the air. This dust when not stirred up gradually settles, and when the 

 germs chance to sow themselves in good ground, with the temperature 

 neither too hot nor too cold, they will immediately begin to grow and 

 multiply, generally at a prodigious rate, living on the material and 

 bringing about its conversion into new and usually simpler forms of 

 matter. 



The inference that putrefaction has a similar origin naturally sug- 

 gests itself. We know that meat during warm weather rapidly becomes 

 putrid. Such a piece of meat examined under a powerful microscope 

 will be found to be swarming with bacteria. Now, it is found that 

 exposure to the temperature of boiling water if sufficiently prolonged — 

 for some bacteria die harder than others — will kill them, and the freez- 

 ing temperature will render them inactive, though without always 

 destroying them; that certain so-called antiseptics, carbolic acid, cor- 

 rosive sublimate, boric acid, etc., act as poisons and kill them. We 

 can recall for ourselves a number of instances where one or another of 

 these methods is employed to prevent putrefaction and decay. Meat 

 and milk are preserved by heating them in air-tight tins. In summer 

 time milk may be kept from turning sour by boiliug it, and game pre- 

 served untainted by parboiling it. In a similar manner cool larders 

 and refrigerating chambers retard or prevent putrefaction. 



