Fluorine

I had always heard that fluorine was a brownish-yellow gas, but I always wondered how anyone knew. You cannot keep fluorine in a glass container since the glass would burn. If you keep it in a copper cask, you can't see it. And if you tried to see it outside a container, you would die from inhalation. But today I was browsing the internet and found the first picture of fluorine gas I have ever seen, shown left. For the story of how this sample was prepared, see here