If a professor who inspected it personally thinks it contains mercury then that is further support for the idea. I seriously doubt that it contains uranium or platinum, though I could see mercury being used to surround highly radioactive substances to absorb and block radiation. This item appears to be a simple cast iron container for mercury however. Mercury metal is somewhat valuable, worth about 100 USD per pound (454 grams) but is difficult and dangerous to handle and ship. Gold miners in your area would be the best to sell it to. Old containers of mercury are also filled with mercury gas/vapors which is very toxic to breathe so don't open it without proper precautions.
I want to tell you that I am very happy that you are here sharing your discoveries and hunts in Tanzania, we mostly see items from the North America and Europe but very little from Africa so it is very interesting to see and read what you have to share on the forum here. I personally would like to see more antique bottles from the old dump you found, aside from good scrap metal like copper and brass that was often just thrown away in the past instead of sold for scrap, you can find special rare bottles valuable to collectors and items thrown away accidentally like coins and silver spoons and other silver items. I dig dumps for antique bottles here in the USA and have found gold items as well as over a pound of sterling silver. There is also a chance that antique African artifacts could be recovered from the dumpsite that have value now but which were not worth much 100 years ago.
Can you measure its weight?
