86 ELEMENTS OF GENERAL SCIENCE 



much about bacteria must give his attention to this branch 

 of science alone. On account of the danger to man from 

 disease-producing bacteria, the science of bacteriology has been 

 taught largely in connection with the study of medicine, but 

 its importance in other respects is coming to be more commonly 

 recognized. 



94. Relation of bacteria to decay. Since it is chiefly bacteria 

 which cause the change in organic substances known as decay, 

 one way to prevent the decay of anything is to prevent the 

 growth of these organisms in it. This may be done either by 

 keeping them out of the substance or by adding to it something 

 that is fatal to them and so making their growth impossible. 



The latter method is sometimes used to preserve milk, 

 formaldehyde being added in small quantities for this pur- 

 pose. Formaldehyde is quite effective in preserving the 

 milk, but it is very objectionable in a food material. Other 

 chemicals are used in canned goods for the same purpose, but 

 in general it may be said that any chemical which will de- 

 stroy bacteria is not likely to be a suitable article of food. 

 This objection does not apply, however, to the preservation 

 of things by means of salt, sugar, or vinegar. 



In every way the most successful method of preventing 

 harm by bacteria is to kill those present and prevent the en- 

 trance of others. This is practiced in the canning of fruits and 

 other food products. Since bacteria are sure to be in the fruit 

 at the outset, it is first boiled in order to kill them. It is 

 then sealed up air tight while still hot, in order to exclude all 

 dependent plants, and if both operations have been success- 

 ful (that is, if there are no living things in the can and no 

 opportunity exists for them to get in), the fruit will keep 

 indefinitely. 



The process by means of which all life of every kind is 

 destroyed by heat, chemicals, or other means is known as 

 sterilization. The decay of substances may also be hindered 

 through drying or refrigeration. 



