In several of the Doc Savage stories, Doc is said to have discovered a gas which had "the lifting qualities of hydrogen and the non-inflammable merits of helium" which he uses in his dirigibles. This always disturbed me even as a teenager. Hydrogen is the lightest element in the periodic table with an atomic weight of 1.0079 grams per mole of atoms, and in its normal diatomic gaseous form (H2), it has an atomic weight of 2.0158 grams per mole of molecules. Helium (He) is the lightest of the inert gases with an atomic weight of 4.0026 grams per mole. These are the two lightest and simplest elements on the periodic table. Hydrogen contains one proton, and one electron ordinarily (Protium) and has some rare isotopes which contain one neutron (Deuterium) or two neutrons (Tritium). Helium has 2 protons and 2 electrons with from 1 to 2 neutrons in stable isotopes. Helium-3 (HE-3) is the lightest stable inert gas with an atomic weight of 3.0160. There can be no normal matter that is lighter than these 2 elements. For this reason, Philip Jose Farmer in Doc Savage: His Apocalyptic Life dismisses this as a fanciful claim with no real merit.
But the key word is "normal matter". There do exist exotic forms of matter in which one or another of the subatomic particles are replaced with a different particle. For example there can be mesic atoms in which the Mu-meson replaces the electrons. This is a short lived variant which decays rapidly in nature. And the Mu-meson weighs 209 times as much as the electron.
But there is a form of exotic matter which weighs far less than normal matter. It is Positronium (Ps). This material occurs when a gamma ray passes close to a massive nucleus and undergoes "pair-production" resulting in one normal electron and one anti-matter electron also called a positron. If the dynamics of the reaction are just right, the two particles form an exotic "atom" with the positron in the center and the electron in a shell around it. This Positronium atom is similar in configuration to a hydrogen atom but is far less massive. Since it has only an electron and a positron both of which have a mass that 1/1836 times that of the proton, the total mass of a basic Ps atom is only 0.00109 grams per mole or only a little more than one tenth of one percent the mass of monatomic hydrogen.
Now, suppose someone was able to generate Positronium with 2 positrons, 2 electrons, and a single neutron to stabilize the nucleus. The resulting configuration would be chemically inert like helium but it would have a mass of 1.00218 which is actually less than that of monatomic hydrogen! This would meet the criteria that are specified in the Doc Savage Sagas.
So I propose that the gas which Doc Savage concocted was actually Positronium in a Helium-3 configuration. This represents a major advance in nuclear chemistry on an industrial scale and hints that Doc must have had some advanced theoretical and technical knowledge about nuclear physics.