THE FRUIT JAR 177 



for keeping out all microorganisms (Fig. 65). The sig- 

 nificant feature of these fruit jars is the rubber ring a, 

 which is clamped tightly upon a flat ledge on the jar c, by 

 means of the cover b, so made that heavy pressure can be 

 exerted upon the rubber. This pressure upon the rub- 

 ber effectually excludes all air and all bacteria. Fresh 

 rubber rings should be used each time the jar is filled, 

 since the efficiency of the sealing depends upon the soft- 

 ness and elasticity of ^ ==== . 

 the rubber; if this 

 gets hard, as it will 

 in a few months, the 

 sealing will not be 

 effectual. Of course 

 the whole jar must 



be Sterilized before FIG. 65. The top of a common fruit jar. 



being filled. To do At a is the rubber rin s u P on which the suc ' 



cess of the sealing is dependent. 



this it is best to place 



it in cold water, bringing the water to a vigorous boil, and 



then fill the jar while it is still hot. 



The glass fruit jar is almost universally used in the 

 home, is very convenient, and can be used again and 

 again. But in canning factories the use of tin cans is 

 largely adopted, since they are less expensive and are 

 to be used but once. The principle of their use, how- 

 ever, is exactly the same as that of the glass jar, although 

 the details are different. The material to be canned, 

 with or without previous boiling, is put in the tin can, 

 upon which a cover is placed and sealed firmly by sol- 

 dering, the whole now being closed to the air except 

 for a small opening in the cover. Then the can, with 



