654 THE POPULAR SCIENCE MONTHLY. 



The second suggestion, methods of preparing and preserving 

 food so that it shall be free from poisonous and harmful sub- 

 stances, indicates the necessity for some knowledge of bacteri- 

 ology. The various fermentative and putrefactive changes which 

 take place in food substances are caused principally by the growth 

 in them of microscopic forms of plant life known by the general 

 name of micro-organisms. 



When micro-organisms grow in masses, as may be seen in the 

 green and yellow molds of bread and cake, they are plainly 

 visible to the naked eye, but to distinguish individuals a micro- 

 scope of high magnifying power is necessary. The common mil- 

 dew, the decay of apples, melons, and other fruits, the rot of vege- 

 tables, and the decomposition of eggs and meat are due to the 

 transforming power of these invisible agents. One of the most 

 common and best known is yeast, which has been more studied 

 and is probably better understood than any of the ferments. It 

 is frequently mentioned to illustrate the transforming power of 

 these infinitely tiny forms of life. A bit of yeast is like a little 

 mass of seeds, each a single cell ; these, when they are placed in a 

 proper medium in other words, find the surroundings of food, 

 moisture, and warmth necessary for their life multiply with 

 extraordinary rapidity, using what they require of the food in 

 which they find themselves, decomposing sugar and starch and 

 establishing changes which result in carbonic acid and alcohol as 

 the chief products. We take advantage of the production of car- 

 bonic acid by yeast to make our loaves of light and wholesome 

 bread. 



Micro-organisms are everywhere : they exist in the earth and 

 the sea ; in plants and animals ; on the surface of our bodies and 

 in the digestive canal ; in cooked and uncooked food ; in refuse, 

 particularly animal waste ; on our clothing, books, furniture, and 

 in the dust of the atmosphere. Wherever they find suitable food, 

 warmth, and moisture they increase with wonderful rapidity, and, 

 if undisturbed would in time completely transform the object 

 upon which they fall. However, by removing any one of the fac- 

 tors necessary to their growth, they cease to multiply, and under 

 such conditions some species remain inert, some die. 



Like other forms of life, micro-organisms by their growth 

 give rise to various products which may be either harmless or 

 harmful. Of the latter may be mentioned noxious gases which 

 pollute the air and poisonous substances which render our food 

 unwholesome. The souring of milk, and the putrefactive changes 

 which, in the presence of heat, so rapidly set in in eggs, meat, 

 oysters, lobsters, crabs, and other albumen-containing foods, are 

 among the results of their transforming power. Perhaps the 

 most important point for us to consider here is, that it is highly 



