274- 
O. 
Ochre, red, description and uses of, 199 
Obsidian, description and uses of, 66 
Oil, rancidity of, how corrected, 207 
Onyx, description and use of, 57 
Opal, description of, where found, use and value of, 63 
, imitations of, 64 
, Hydrophanous, singular quality, use, and value of, 64 
', Common, 65 
, Mother of pearl, and wood opal, 65, 66 
Orpiment, yellow, description and uses of, 214 
, red, 214 
Oxides, 11 
Oxygen, 10 
gas, 11 
P. 
Paint, white, how to prevent pernicious effects of, 205 
, made of oxide of zinc, 211 
Paviour's flags, of what made, 234 
Paving stones, 225, 233 
Pearl-ash, what it is, and how prepared, 136 
white, of what made, and use of, 218 
Pebbles, 46 
Petrifactions. See Tufa, 
Petrifying springs, at Knaresborough and Matlock, 113 
Petroleum, description, properties, and uses of, 157 
Petunze, 69 
Pewter, of what made, 202 
Phosphats, 12 
Phosphoric acid, 12 20 
Phosphorus, 19 
Pinchbeck, of what made, and use of, 187 
Pipe-clay, description and use of, 72 
Plaster of Paris, of what made, and use of, 118 
Plate glass, how made, 56 
Plating of copper, &c. how performed, 180 
Platina, description, properties, and uses of, 166, 167 
Pompey's pillar, t>f what formed, 230 
Ponderous earth. See Barytes. 
Porcelain-clay, description and use of, 74 
, Chinese, French, and English porcelain, account of, 74 
, how manufactured, glazed, and painted, 74, 75 
, biscuit, how made, 47, 75 
Porphyry, description and uses of, 229 
Portland stone, description and use of, 114 
Potash, description of, how obtained, and use of, 135 
Potstone, description and use of, 90 
