I can remember my parents having one in the 70s and it used to fascinate me.
I bought one in my 20s and it had the same effect on me.
Whilst clearing out the house last week we discovered the one from my 20s, but it had no base to it for some reason (my wife inexplicably chucks out lots of things at times).
I won a second hand base on ebay, but then discovered that the wax in the glass bulb had separated and try as much as I could I was unable to mix the high and low density waxes back together.
My children were quite excited after me finding the old glass bulb and then winning the base for it, but then the disappointment set in when it didn’t work.
Mathmos do a replacement glass bulb for 23.95 delivered, doesn’t seem too bad for hours of fun staring at coloured blobs of wax rising and falling does it?