"How is it possible to can in a jar?" Commercial food preservation began shortly after Francois Appert developed the boiling water method. Although glass jars were often used initially, they were difficult to seal and expensive to ship. The first tin canister was patented in 1810 at approximately the same time that Appert developed his method. Commercial canning in tinplate canisters soon became common, because it was easier, less expensive and safer to preserve food in the tin canister. Soon the word "canister" was shortened to can and canning became a common term for commercial food preservation. The term stuck even after John L. Mason invented the jar with the threaded screw and matching lid. With Mason's invention, there was an easy, effective and safe way for the homemaker to preserve food in glass.
Another term for food preservation is "putting up." That term may have originated because the homemaker canned her food, then "put up" the jars on a shelf in a darkened room. Food can also be laid down; e.g., meat is laid down after salting or brining. "So it has been possible to can in Ball jars, …, or to put things up in them, even after the contents had been laid down."
Sources: William F. A Brantley's "Collector's Guide to Ball Jars," Rosemary Humbert Martin, publisher, Muncie, Indiana, (C) 1975; George E. Myers, "The Story of Ball Brothers," Volume 1, 1965.