282 On Repulsion resulting from Radiation. [Feb. 10, 



Rock-salt and glass were next tested against each other in vacuo in a 

 torsion-balance. Professors Dewar and Tait say that rock-salt is inactive 

 when the beam from a candle is thrown on it, while a glass disk is active. 

 The author has failed to corroborate these results ; he found the mean of 

 several concordant observations to be — rock-salt 39, glass 40. 



The Measurement of the Force. 



The author describes a torsion-balance in which he is enabled to weigh 

 the force of radiation from a candle, and give it in decimals of a grain. 

 The principle of the instrument is that of W. Ritchie's torsion-balance, 

 described in the Philosophical Transactions for 1830. The construc- 

 tion is somewhat complicated, and cannot be well described without refer- 

 ence to the diagrams which accompany the original paper. A light beam, 

 having two square inches of pith at one end, is balanced on a very fine 

 fibre of glass stretched horizontally in a tube, one end of the fibre being 

 connected with a torsion-handle passing through the tube, and indicating 

 angular movements on a graduated circle. The beam is cemented to the 

 torsion-fibre, and the whole is enclosed in glass and connected with the 

 mercury -pump and exhausted as perfectly as possible. A weight of 

 0*01 grain is so arranged that it can be placed on the pith or removed 

 from it at pleasure. A ray of light from a lamp reflected from a mirror 

 in the centre of the beam to a millimetre- scale 4 feet off shows the 

 slightest movement. When the reflected ray points to zero, a turn of 

 the torsion-handle in one or the other direction will raise or depress the 

 pith end of the beam, and thus cause the index ray to travel along the 

 scale to the right or to the left. If a small weight is placed on one end 

 so as to depress it, and the torsion -handle is then turned, the tendency 

 of the glass fibre to untwist itself will ultimately balance the downward 

 pressure of the weight, and will again bring the index ray to zero. It 

 was found that when the weight of the yj-^ of a grain was placed on the 

 pith surface, the torsion-handle had to be turned 27 revolutions and 

 353°, or 10073° before the beam became horizontal. The downward 

 pressure of the T ^j of a grain was therefore equivalent to the force of 

 torsion of the glass thread when twisted through 10073°. 



The author next ascertained what was the smallest amount of weight 

 which the balance would indicate. He found that 1° of torsion gave a 

 very decided movement of the index ray of light, a torsion of 10073° 

 balancing the jfo of a grain, while 10074° overbalanced it. The 



99 



balance will therefore turn to the i 00 000 00Q of a grain. 



Divide a grain weight into a million parts, place one of them on the 

 pan of the balance, and the beam will be instantly depressed. 



"Weighed in this balance the mechanical force of a candle 12 inches off 

 was found to be 0*000444 grain ; of a candle 6 inches off 0-001772 grain. 

 At half the distance the weight of radiation should be four times, or 

 0-001776 grain ; the difference between theory and experiment being 



