that close with difficulty, especially if the tops screw on, are not
likely to keep food successfully because the bacteria in the air will
have a chance to enter and thus cause the food to spoil.

Glass jars used for canning foods have improved with canning methods.
The old-style jars had a groove into which the cover fit, and melted
sealing wax or rosin was poured into the space surrounding the cover.
Later came the screw-top jar shown in Fig. 3. This type of jar has been
extensively used with excellent results. Both the mouth of this jar and
the jar top, which is made of metal, usually zinc, lined with glass or
porcelain, have threads that match, and the jar is sealed by placing the
jar rubber over the top, or ridge, of the jar and then screwing the jar
top firmly in place. Such jars, however, are more difficult to make
air-tight than some of the newer types. One of these jars is illustrated
in Fig. 4. It is provided with a glass cover that fits on the ridge of
the jar and a metal clasp that serves to hold the cover in place and to
make the jar air-tight after a rubber is placed in position. Another
convenient and simple type of glass jar, known as the _automatic seal
top_, has a metal cover with a rubber attached.

Another improvement in jars is that the opening has been enlarged so
that large fruits and vegetables, such as peaches, tomatoes, etc., can
be packed into them whole. With such wide-mouthed jars, it is easier to
pack the contents in an orderly manner and thus improve the appearance
of the product. Besides, it is a simpler matter to clean such a jar than
one that has a small opening.

[Illustration: FIG. 4]

23. JAR TOPS AND COVERS.--While the tops, or covers, for glass jars are