Mendeleev-D: central claims
No source gap: the essay and the review article have full text layers (8 OCR pages are blank or tables; the periodic tables on PDF pp.32 and 34 were read from page images). The two copies of the essay are the same 1904 book. The review article by Stewart (2019) is a modern secondary source, now deep-read in full (deep/mendeleev-predictions-bdcaef.md).
- The ether is a real chemical element: a monatomic gas, like helium or argon, that has weight, permeates all bodies and cannot combine chemically (Mendeleev 1902/1904, pp.22-23).
- The periodic system needs a "zero group" of inert gases (He, Ne, Ar, Kr, Xe) and, above it, a "zero series" holding the ether element x, with a second unknown element y (possibly coronium, the green line near 531.7 nm) between x and hydrogen (pp.28-39). In the 1904 table x stands in series 0 and y in series 1 beside hydrogen (H = 1.008), both printed without atomic weights. The bound x <= 0.17 is a three-point quadratic extrapolation from the noble-gas ratios (He:x = 23.61), reproducible several ways with an 8-fold spread in x. Names as in the English text: Kirpitchnikoff (Russian Kirpichev), Rostovsky (Russian Rogovsky); coronium in volcanic gas is Nasini, Andreoli and Salvadori 1898. Related contemporary view: Werner's 1905 table also left spaces for elements lighter than hydrogen (Stewart), and Mendeleev cites Dewar's 1902 Belfast address as kindred (p.51).
- The ether atom has an atomic weight of about one millionth of hydrogen's and moves at about 2,250 km/s, derived from kinetic theory at an assumed space temperature of -80 degC and from escape velocities of Earth, Sun and massive stars (pp.43-51). The printed limits on x run from 9.6e-7 down to 5.3e-11; the adopted 'nearly one millionth' is the upper edge of that range. It rests on the single assumed temperature (x scales with T; about 70 times smaller at 2.7 K) and does not use the x <= 0.17 periodic bound. The speed is the rms speed (1843 m/s scale; mean speed 1694 m/s form), and 2,250 km/s exceeds the Sun's escape speed (608 km/s) by a factor of 3.7.
- Radioactivity (U, Th, Ra as ether-accumulating "suns"), the Curie zinc-sulphide fluorescence demonstration and Dewar's low-temperature phosphorescence are offered as supporting phenomena for the ether (pp.52-58).
- Matter has no single primary substance and atoms are not divisible into electrons or transmuted; only the ether substance is a unity (pp.13-17, 40-41). His stated reason is a distinction 'between matter, force, and mind' (pp.40-41).
- (From Stewart 2019, secondary) Mendeleev's earlier periodic predictions (gallium, scandium, germanium, and several others) were largely successful, while the ether and coronium additions were not predictions from element properties. Stewart's counts: ten element predictions named in the body (the abstract says eleven); eight successes, one multiple error, two irrelevancies. Stewart's statement of 'six more blanks between hydrogen and helium' is not shown by his own Fig. 2. Context from Stewart (his characterization): from 1872 to 1881 Mendeleev's gas-elasticity programme aimed at the nature of the ether, and he instigated the May 1875 spiritualism commission; he was nominated for the 1906 Nobel Prize, lost 5 votes to 4 to Moissan.