Lothar Meyer, in 1868, and Mendeléeff, in 1869, published atomic weights showing improvements in the theories of valency and the interrelationship of atomic weights. Mendeléeff was able to predict from the vacant positions in his table the discovery of important new elements. A number of these elements have since been discovered. The aniline dye industries have grown out of the discoveries of many chemists. The basic work was done by Faraday, Laurent, and Runge, who isolated valuable hydrocarbons from coal gas tar. Hoffmann discovered aniline and Perkin obtained mauve in 1856 by the oxidation of aniline with chromic acid. It was this and subsequent discoveries by Perkin which gave the greatest impetus to synthetic dyes. The solubility of a dye was improved by increasing its acidity (sulphonation) or by increasing its alkalinity (alkylation). Similar dyes are now made by the same methods from many common aromatic substances. The chemistry of explosives was developed by Van Helmont, Debus, Bunsen, Abel, Nobel, and, others. Fulminates were used for detonators by Ure in 1831, picrates were employed as explosives by Fontaine and Abel; nitrocellulose (guncotton) discovered by Braconnot in 1832 and used as an explosive by Schönbein in 1846, and nitroglycerine was produced by Sobrero in 1847. Smokeless powders made from guncotton, dynamite, and gelatine were introduced by Nobel in 1890. Pasteur showed, in 1848, that when the double sodium ammonium racemate was crystallized, two kinds of crystals separated from the solution. When one set of crystals was dissolved in water the solution rotated a beam of polarized light to the left, while the aqueous solution of the other crystals rotated the light to the right. These crystals thus revealed their geometrical properties with perfect light while in solution in water. Pasteur noted that optical activity of this kind is the expression of some form of molecular asymmetry. Le Bel in 1874 also pointed out that optical activity is an expression of the asymmetry of the chemical molecule and showed that all carbon compounds which are optically active contain a carbon atom combined with four different atoms, or groups. Van't Hoff showed in 1875 that there were definite relations between the arrangements of tetrahedral carbon atoms and polarization phenomena and established the theory of such atoms. Willard Gibbs, of Yale, discovered what is known as the phase rule, which shows, by thermodynamic methods, how the conditions of chemical equilibria can be systematically grouped.