PRESERVATION OF FOODS 23 



rats and mice, and the varipus insects in the home are 

 only too familiar pests. But not every one understands 

 that in addition to these large animals there is a great 

 host of plants and animals which seize every opportunity 

 of feeding upon that which we intend for our own use. 

 All such small animals and plants go by the general name 

 of microbes or microorganisms. 



We are chiefly concerned, in this book, with three 

 important groups of plants. Some of these plants are 

 large enough to be seen easily and are generally well 

 known, such as the molds that occur everywhere, and are 

 always regarded as nuisances in household economy. In 

 addition to the visible plants there is a still larger num- 

 ber of others, quite too small to be visible to the naked 

 eye and, indeed, only seen with the high powers of the 

 microscope. These invisible organisms are the smallest 

 living beings of which we have any knowledge, and are 

 both friends and foes. Not only are they invisible to the 

 naked eye but to the ordinary housewife they are quite 

 unknown. Until within recent years they have been 

 unknown even to scientists, and although science has now 

 learned to understand quite well what they are and what 

 they do, to the public in general they are little more than 

 a name around which cluster various mysteries and in 

 regard to which there is no general information. Molds 

 and yeasts have long been known. The term bacteria refers 

 to organisms which began to claim public attention much 

 more recently but in regard to which there is at present a 

 large amount of misunderstanding. Even though these or- 

 ganisms are very minute, and though she knows little about 

 them, the housewife finds them the most serious, indeed 



