What is the lightest substance known?

The element hydrogen is the lightest substance known to science. Its specific gravity, compared to air, is 0.0695. Helium, with a specific gravity of 0.139, is the next lightest substance. Specific gravity is relative density, that is, the ratio of the weight of any volume of a substance to the weight of an equal volume of some other substance used as. a unit or standard. Air is generally used as the standard for -gases, and water for solids and liquids. When we say that the specific gravity of helium is 0.139 and that of hydrogen 0.0695, ^s expresses the fact that helium is twice as heavy as hydrogen. Although helium has twice the weight of hydrogen, its lifting power when used in balloons is about 93 per cent that of hydrogen. The heaviest known substance is osmium, a rare platinum-like metal with a specific gravity of 22.5. Iridium, with a specific gravity of 22.42, comes second; platinum, 21.37, third; and gold, 19.33, fourth. Osmium, the heaviest known substance in the world, weighs more than 250,000 times as much as hydrogen, the lightest substance. Lithium, a silver-white soft element with a specific gravity of 0.53, is the lightest metal known.