I just stumbled across this word today, and was baffled to find that a thermos is a "magic bottle"? What reason is there for the kanji chosen? Was it a decision by a company who introduced/manufactured the first thermoses in Japan?
We had not understood the reason why a thermos is called 魔法瓶, but Mr.Awazu, who was a director of 魔法瓶 memorial house, found out the reason a few years ago.
He found out a oldest newspaper article which was written the word 魔法瓶 at October 11th in 1907. The article said that in his interview about hunting, a person referred to the high performance and convenience of a thermos, which had been just imported to Japan at that time, and quoted 魔法瓶(Magic Bottle) in Aesop’s Fable. His words were used in the oldest advertisement of a thermos in Japan and he was decided to be the author of 魔法瓶.
However 魔法瓶(Magic Bottle) isn't in Aesop’s Fable and it seems to be his misunderstanding.
This is the source. http://blog.livedoor.jp/blog_channel-0004/archives/27866247.html