XVIII. On Repulsion resulting from Radiation . — Part II. 
By William Crookes, F.R.S. &c. 
Received March 20, — Read April 22, 1875. 
Contents. 
Par. 
Introduction 81 
Improvements in the Sprengel pump 82 
Horizontal index in exhausted bulb 84 
Experiments with horizontal index 87 
Action of hot and cold bodies on each other . . 88 
Experiments on attraction at high pressures . . 95 
Action through various screens 97 
Pendulum apparatus with magnesium boh .... 99 
Action of radiation on pendulum in vacuo .... 100 
The horizontal torsion-apparatus 102 
Selection of the suspending glass fibre 103 
Experiments with the horizontal torsion- apparatusl 04 
a. Effect of residual gas 105 
Par. 
b. Action of continued radiation 106 
c. Action of intermittent radiation 107 
d. Repulsion through various screens 109 
e. Action of the electric and solar spectrum . . 110 
/. The neutral point 112 
Double torsion-apparatus 115 
Simultaneous attraction and repulsion at same 
pressure 115 
Certain hypothetical explanations of the pheno- 
mena 117 
a. The air-current' theory 119 
b. The electrical theory 120 
c. The evaporation and condensation theory . . 122 
81. The present paper is in continuation of one which I had the honour of reading 
before the Royal Society, December 11th, 1873, and which was published in the Phi- 
losophical Transactions, vol. clxiv. part 2, page 501. In that paper I described various 
pieces of apparatus, chiefly in the form of delicate balances suspended in glass tubes, by 
means of which I was enabled to show attraction or repulsion when radiation acted on 
a mass at one end of the beam, according as the glass tube contained air at the normal 
pressure, or was perfectly exhausted. At an intermediate internal pressure the action 
of radiation appeared nil. Towards the end of the paper I said (70), “ I have arranged 
apparatus for obtaining the movements of repulsion and attraction in a horizontal 
instead of a vertical plane. Instead of supporting the beams on needle-points, so that 
they could only move up and down, I suspend them by the centre to a long fibre of 
cocoon-silk in such a manner that the movements would be in a horizontal plane. 
With apparatus of this kind, using very varied materials for the index, enclosing them 
in tubes and bulbs of different sizes, and experimenting in air and gases of different 
densities up to Sprengel and chemical vacua, I have carried out a large series of expe- 
riments, and have obtained results which, whilst they entirely corroborate those already 
described, carry the investigation some steps further in other directions.” 
82. I have introduced two important improvements into the Sprengel pump* which 
* Philosophical Transactions, 1873, vol. clxiii. p. 295; 1874, vol. clxiv. pp. 509, 510. Phil. Mag. Aug. 1874. 
