382 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



withstand the extremes of temperature, and were expensive, bulky, and 

 costly in transportation. In 1825, Thomas Kensett secured a patent 

 on the use of tin cans in preserving food, and in the same year began 

 using the process in his factory. The early manufacture of tin cans 

 was by hand and crude, the bodies being cut with shears and the side 

 seams made with a plumb joint (that is meeting but not overlapping), 

 and then soldered together. Heads were made to set into the body and 

 were soldered in place in a very crude manner. The making of 100 cans 

 was considered a good day's work for one man. Improvements were 

 gradually made, however, in their manufacture, until finally can making 

 became a distinct industry and now all the parts are made and put 

 together by mechanical devices. 



In the original Appert process, the goods were cooked in open kettles, 

 the highest temperature obtainable by this method being the boiling 

 point. A little later common salt was used to aid in securing a higher 

 temperature, and this was followed later by the use of calcium chloride 

 which made possible a temperature of 115. In 1874, a closed kettle 

 was invented for superheating water with steam, and this was immediately 

 followed by another improved kettle in which dry steam was used, the 

 principle employed being that of the modern autoclav, by which method 

 any desired temperature may be obtained and modified to suit the re- 

 quirements of different classes of food. 



ECONOMIC IMPORTANCE. 



FROM STANDPOINT OF HEALTH AND DIETETICS. The value of a 

 variety of foods, especially fruits and vegetables, is recognized by dieti- 

 cians. Unfortunately, however, the season for fresh fruits and vege- 

 tables is comparatively short Moreover, many foods grown exclusively 

 in one section of country will not withstand shipping in a fresh condition 

 to other sections. In spite of improved methods of refrigeration, it is 

 not practicable to ship fresh sea foods to far inland towns, or to send 

 some perishable products of warm climates to cold countries. The 

 canning and preserving industry overcomes these difficulties by supplying 

 pure, clean, wholesome fruits, vegetables, meats, and fish to any region 

 the year round, and at prices comparatively low. 



FROM STANDPOINT OF COMMERCE. In its commercial aspect, the 

 importance of the industry can scarcely be estimated. Canned products 



