Mercury has become a serious toxic waste. It is

Solution

Mercury is a heavy silvery-white metal that is liquid at room temperatures. It is commonly used in thermometers, nano-meters and barometers, thus it is present in nearly every chemistry and physics lab. Confined in such instruments mercury itself is not likely to cause health problems, but if there is a means for mercury vapor to enter the environment, these highly toxic vapors present a health hazard.
A fluorescent lamp tube is filled with a gas containing low pressure mercury vapor and argon, xenon, neon, or krypton. Fish can absorb methyl mercury from their food and directly from water as it passes over their gills.