1877.] 



on Crookes's Force. 



555 



When the gauge of the pump showed a tension of 7 millims., as com- 

 pared with the mercurial column of a barometer standing in the same 

 vessel of mercury, the glass disk was distinctly repelled from the pith 

 and towards the source of light. As the exhaustion was continued the 

 repulsion between the pith and the glass increased. The apparatus was 

 sealed off from the pump when the mercury falling in the exhaust tube 

 had for some days produced a metallic sound. Feeble illumination now 

 caused the glass disk to be forcibly driven a^^'ay from the pith*. 



We now endeavoured to determine quantitatively the influence of 

 variations in the tension of the residual gas, and also the influence of 

 variations in distance between the reacting surfaces. Eor this purpose 

 we constructed the apparatus represented in fig. 2. 



On a wooden stand supported by three levelling-screws rests a glass 

 tube 20 centims. in length and 3-8 centims. in diameter, having a tubular 

 opening at one side, into which is cemented horizontally a smaller tube 

 1-5 centim. in diameter. In the larger tube there is a circular disk of 

 elder-pith 2*3 centims. in diameter, ha"vdng one side blackened with 

 lampblack ; it is supported in a vertical position on a movable stand of 

 iron wire. By means of a magnet the pith disk can be moved up and 

 down the tube, and thus placed at any required distance (within 

 12 centims.) from a delicately suspended circular disk of thin microscope- 

 glass, 3 centims. in diameter and 0'3 millim. in thickness. The glass 

 disk is attached to the end of a glass arm, which is suspended in the 

 smaller tube by means of a silk cocoon fibre contained in a vertical limb 

 38 centims. in length and 9 millims. in diameter. In order that the 

 torsion of the silk fibre may be conveniently regulated, there is a small 

 fl-shaped piece of iron wire attached to it a few centimetres below the 

 end from w^hich it hangs. A horseshoe magnet is suspended outside 

 the tube with the piece of wire between its poles. By turning the 

 magnet round torsion may be imparted to the silk fibre. The balance of 

 the glass arm is adjusted by means of a small iron ring which it carries ; 

 the position of the ring can be altered at will by an external magnet. 

 There is a small silvered mirror attached to the arm at the point of suspen- 

 sion ; this reflects the image of a narrow illuminated slit on to a scale divided 

 into degrees 2-5 millims. each. An alteration in the position of the 

 index amounting to 0*5 millim. is readily observed; this corresponds 

 with a change in the position of the outer edge of the glass disk amount- 

 ing to 0-033 millim. One end of the large tube is ground perfectly flat 

 and closed by cementing to it a plate of glass 4 millims. in thickness ; 

 through this light is admitted to the pith disk by an arrangement to be 

 presently referred to. The other end of the large tube is contracted and 

 terminates in a narrow tube bent upwards, partly packed with gold leaf 

 (to intercept mercury vapour), and attached to the exhaust-tube of a 



* The apparatus was sealed off on the 14th of Api'il, 1876. The experiments 

 described above were made in Mai'ch. 



