l, 181.

Pitch of sound, 280, 281.
cause of, 282.
in wind instruments, 296-299.

Plane, inclined, 162-166.

Plants, and atmosphere, 55.
and light, 131-132.
and nitrogen, 261.

Plate developing, photographic, 128.

Pneumatic dispatch tube, 205.

Poles, magnetic, 330-332.
of cell, 308.

Positive electrode, 308.

Potash, in fertilizer, 263-265.

Potassium chlorate and oxygen, 46.
permanganate, 100.
tartrate and Rochelle salt, 227.

Power, candle, 105-107.
electric, 351.
horse, 173, 351.
sources of, 174, 175, 185.
transmission by belts, 171.
water, 176-180.

Precipitation, 72, 73.

Preservatives, food, 252.
wood and metal, 253-254.

Pressure, atmospheric, 82-86.
calculation of atmospheric, 83, 84.
calculation of gas, 92, 93.
calculation of water, 94.
gauge, 92-94.
of illuminating gas, 93.
relation of pressure of gas to volume, 95, 96.
water pressure, 208-211, 214-216.
within the body, 86.

Primary colors, 135.

Print, photographic, 131.

Printing, color designs in cloth, 248, 249.
electrotype, 317.

Prisms, 135.
refraction through, 117.

Proteids, 66.

Pulleys, 166-169.
applications of, 169.

Pump, 187-205.
air, 201-205.
force, 192, 193.
lifting, 189-192.

Pupil of the eye, 122.

Pure food laws, bleaching, 242.
preservatives, 252.

Purification of water, 77, 196.

Push button, 321.

Radiator, 19-21.

Railroads, grading of, 165-166.

Rain, 36, 37.

Rainbow, 134.

Rain water, 225.

Reflection, angle of, 110.
of light, 109-112.
of sound, 278, 279.

Refraction, angle of, 114.
by atmosphere, 114.
of light, 113.
uses of, 115-116.

Relay, telegraph, 325.

Reservoir, 214.
artificial, 211.
construction of, 214-216.
natural, 211.

Resistance, electrical, 312.
internal, of cell, 343.
unit of, 345.

Resonance, 276.

River, volume and value of, 180.

Roads, application of inclined plane to, 165-166.

Rochelle salt, 227, 231.

Rocks, effect of freezing water on, 42-43.
water as a solvent, 71.

Rosin, obtained by distillation, 35.

Safety matches, 47-48.