made of both metal and glass, as has been stated, the glass tops meet
with most favor. Of course, they are breakable, but they are even more
durable than metal tops, which are usually rendered less effective by
the bending they undergo when they are removed from the jar. Covers made
of zinc are being rapidly abandoned, and it has been proved that the
fewer the grooves and the simpler the cover, the more carefully and
successfully can it be cleaned. For safety, glass tops that have become
chipped or nicked on the edges that fit the jar should be replaced by
perfect ones. The covers for automatic-seal jars must be pierced before
they can be removed, and this necessitates a new supply for each
canning. If there is any question about the first-class condition of jar
covers, whether of metal or glass, tops that are perfect should
be provided.

24. JAR RUBBERS.--Jar rubbers are required with jar tops to seal jars
air-tight. Before they are used, they should be tested in the manner
shown in Fig. 5. Good jar rubbers will return to their original shape
after being stretched. Such rubbers should be rather soft and elastic,
and they should fit the jars perfectly and lie down flat when adjusted.
A new supply of rubbers should be purchased each canning season, because
rubber deteriorates as it grows old. Rubbers of good quality will stand
boiling for 5 hours without being affected, but when they have become
stiff and hard from age it is sometimes impossible to make jars
air-tight. Occasionally, two old rubbers that are comparatively soft may
be used in place of a new one, and sometimes old rubbers are dipped in
paraffin and then used. However, if there is any difficulty in sealing
jars properly with rubbers so treated, they should be discarded and good
ones used.

25. TIN CANS.--For household canning, tin cans are not so convenient as