320 AGRICULTURAL BACTERIOLOGY. 



The plan commonly adopted is well known. The material 

 to be preserved is placed in cans of some sort and then heated 

 very hot. Sometimes a temperature of boiling alone is suf- 

 ficient, and sometimes a temperature above boiling is obtained 

 by the use of steam under pressure. After an amount of heat- 

 ing, supposed to be sufficient to destroy all bacteria, the can 

 is sealed hermetically, before it has cooled down enough to 

 allow a secondary contamination from outside bacteria. When 

 the process is successful in destroying all bacteria, even to the 

 very last spore, and the sealing takes place properly, the food 

 may be preserved indefinitely without the slightest tendency to 

 undergo putrefactive changes. There is apparently no limit to 

 the length of time that properly canned food products may be 

 preserved. 



The development of the canning industry does not belong 

 to our immediate subject, but there are certain facts connected 

 with the matter which have produced great changes in the 

 possibilities of agriculture. 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 can- 

 ning 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 another farm product. It is no 

 longer necessary for the farmer to depend upon his own 

 market, but, by the process of canning, his market may be the 

 world, open to him the whole twelve months of the year. The 

 canning industry makes it possible for the farmer to become a 

 specialist where it was impossible a few years ago. He may 

 raise green corn, or tomatoes, or strawberries as abundantly as 



