Parametral form, 56 Pasteur’s law, 155; research on morphotropy of tartrates, 100; research on tartaric and racemic acids, 142–150 Penfield’s diagram of spherical projection, 61, 62 _Penicillium glaucum_, destruction of dextro component of racemic acid, 148 Perchlorates and permanganates, isomorphism of, 84, 96 Pfaff, 254 Phases, different solid, 137 Phenol and resorcinol, von Groth’s researches on, 103 Phosphate, ammonium magnesium, 43; sodium dihydrogen, dimorphism of, 80, 81 Phosphates and arsenates, isomorphism of, 73, 74 Phosphorus, dimorphism of, 138 Photomicrographs of growing crystals, mode of obtaining, 41 Pistor’s goniometer, 89 Planeness of crystal faces, 6, 7 Polarisation colours due to optical activity, 179 Polarisation, rectangular, of spectra from doubly refractive prisms, 164 Polariscope, the, 187, 188 Polymorphism, 133–137 Pope and Barlow, theory of, 285–291 Positive and negative uniaxial and biaxial crystals, 166 Potassium bichromate, metastable and labile crystallisation of, 246, 247; manganate, 96; nickel sulphate, 35; selenate, 96; sulphate, 13 Priestley, 23 Primitive form of Romé de l’Isle, 20, 27 Projection polariscope, for convergent light, 92, 186, 188, 189; for parallel light, 201, 202 Progressive change of crystal angles in isomorphous series, 125; of double refraction, 126; of molecular distance ratios, 130, 131; of position of optical ellipsoid, 128 Propyl triphenyl pyrrholone, 106, 108 Proustite, 109, 110 Pseudo-racemism, 153, 154 Pyrargyrite, 109, 110 Pyroelectrical properties of crystals, 149 Quartz, 170; crystalline form of, 171, 172; crystals in rock sections, polarisation colours of, 175–178; crystals, liquid cavities in, 45; crystals on sand grains, 2, 3; double refraction of, 174; interference figure in convergent polarised light, 194–196; optical activity of, 173, 179–181, 210; plates, preparation of for polariscope, 181; polarisation colours of due to optical activity, 209, 210; refractive indices of, 174; screw point-systems of, 151; 60°-prism experiment with, 165, 168; Steno’s research on, 16; twinning of, 215, 216, 219–221, 225; two varieties of as examples of mirror-image symmetry, 171 Racemate, sodium ammonium, Pasteur’s researches on, 147, 148 Racemic acid, 142–146, 233, 234 Racemic forms and racemism, 150, 153, 233 Rammelsberg and morphotropy, 101 Rational indices, 24, 50, 59, 116 Reflection of light by crystal faces, 8 Refractive index, meaning of, 167 Reinitzer, 258 Reusch’s artificial quartzes, 198, 199 Rhombic system, 39; axes and axial planes of, 52, 54 Rhombohedron and its axes, 10, 11, 54, 55, 172 Rings and brushes, optic axial, 190, 191 Rock-salt, cube of in quartz cavity, 45; 60°-prism experiment with, 162, 168 Romé de l’Isle, 18–20; researches on alums, 77 Roozeboom, 238