PRESERVATION BY CANNING 265 
true of corn, peas, and beans, since these materials contain such 
resisting spores that it was once thought that they could not be 
successfully canned. But modern methods of applying heat 
above the boiling temperature have made it possible to sterilize 
thoroughly even these resisting substances, and their canning is 
now periectly feasible. To-day temperatures of 230° to 250° 
are commonly used for such materials. In canning fruit, as a 
rule, such high heat is not needed, since fruits are more often 
spoiled by yeasts and molds than by spore-bearing bacteria, and 
these organisms are easily killed by simple boiling, or by even 
less heat. 
The second step is the sealing. This consists simply in closing 
the vessel containing the sterilized mass in such a way as abso- 
lutely to exclude the air and with it all microdrganisms. It is 
sometimes done in tin cans, when a small hole is left in each can 
so that the air and steam may escape during the sterilizing, after 
which process the hole is sealed by a drop of solder. Sometimes 
glass jars are used, and these are sealed by covers pressed forcibly 
down upon a soft rubber ring. The principle is the same in 
either case. The sterilized food thus removed from any possible 
means of contamination will keep indefinitely. 
The development of the canning industry does not belong to 
our immediate subject, but certain facts connected with the 
matter have produced great changes in the possibilities of agri- 
culture. It has made possible the utilization of a great quantity 
of food products which otherwise could not be used. Certain 
of our fruits are extremely palatable but very perishable, and if 
it were necessary to use them fresh or in a dry condition, only 
comparatively small quantities could be raised. For example: 
before the beginning of tomato canning, only a very small crop 
could be utilized; but the opening of this canning industry has 
entirely changed the conditions, and now great tracts of land 
can be devoted to raising this delicacy, thus opening to the farmer 
an entirely new outlet for his crop. The same is true of many 
