XIII. On Repulsion resulting from Radiation . — Parts III. & IV. 
Ry William Crookes, F.R.S. &c. 
Part III. received January 5, Part IY. received February 5, — Eead February 10, 1876. 
Contents. 
Par. 
Introduction 125 
Action of radiation on black and white surfaces 127 
a. Effect of dark heat 128 
b. Effect of light 129 
Interfering action of aqueous vapour . . 130 
Charcoal vacua 131 
Zollnee’s horizontal pendulum 133 
The bar photometer 135 
The radiometer 143 
Action of dark heat on the radiometer 144 
Eelation between the speed of the radiometer 
and its distance from a standard candle .... 148 
Relation between the speed of a radiometer and 
the amount of light shining on it 149 
Behaviour of a radiometer in a uniformly lighted 
space 151 
Action of coloured light on the radiometer .... 153 
Interfering action of dark heat on the movement 
of the radiometer to light 155 
Action of ice on the radiometer 158 
Negative rotation of the radiometer 159 
Metallic radiometers 163 
Behaviour of radiometer when chilled with ether 165 
Theoretical explanations of the foregoing ano- 
malies ; 170 
Action of ignited platinum spiral on the radio- 
meter 175 
Action of the induction-spark on the radiometer 180 
Eadiometer blacked on both sides 181 
Silvered glass radiometer 182 
One-armed radiometer 183 
Magnetic radiometer for photometric purposes . 184 
Eadiometer for exhibiting motion to an audience 185 
The horizontal torsion-apparatus 186 
Par. 
Apparatus for spectrum observations 187 
Action of the rays of the solar spectrum on the 
torsion-apparatus 189 
Is the effect due to heat or to light ? 192 
Motion-producing value of the different colours 
of the spectrum 193 
Apparent viscosity of a vacuum 194 
Theoretical explanation of the mechanical action 
of radiation 195 
Comparison of the action due to the dark heat- 
rays and the visible rays of the sun 196 
Ditto of a candle 197 
Action of radiation on surfaces of different 
materials and variously coloured 198 
The torsion-apparatus for these observations . . 198 
Comparison between black and white pith .... 200 
Comparison between lampblack and carbonate 
of lead 200 
Comparison between lampblack and iodide of 
palladium 202 
Comparison between lampblack and rock-salt . . 203 
„ „ „ alum 203 
„ „ alum and rock-salt 204 
„ „ rock-salt and glass 207 
„ „ various coloured powders . 208 
The measurement of the force. Eitchie’s torsion- 
balance 209 
Construction of the balance 209 
The torsion-fibre 210 
Measurement of the sensitiveness of the torsion- 
balance 215 
Measurement of the mechanical force of radia- 
tion 216 
o 
z 
MDCCCLXX7I. 
