"Regnirps" wrote in message
...
I would think hydrogen would be an obvious choice. Helium molecules are so
darn
small that they leak out of practically anything. That is why the coated
mylar
party balloons are used instead of latex.

I have never noticed that helium filled latex balloons lose buoyancy any
faster than the Mylar ones. I thought Mylar was used because it is bright
and shiny. I have also heard that birds are less likely to eat it and die,
which probably ranks right up there with urban legends about wedding rice
and birds. Back in the old days (when I was a child) I used to sell balloons
to raise money for Rotary and the local rodeo. No such thing as Mylar
balloons then, but they were filled with helium. The balloons would hold
their helium for a week or more. I also recall latex weather balloons filled
with helium. Although I personally launched literally thousands of toy
balloons filled with helium, I never heard of a bird being killed by one. I
have seen toy helium filled balloons at 11,000 feet.

When we lived in the Philippines in the 70's they still sold balloons filled
with hydrogen, which has been banned in the US because of its flammability.
Let one of these toys near a lit cigarette and you could have one badly
burned kid.