Intensity of Rays of High Ref Tangibility . 217 



cases, the total magnetic force exerted by the permanent 

 magnet on B is an attraction ; and its component along the 



outward-drawn normal is \k-^—dv. 



Thus even with all our assumptions, the force which Jamin 



requires to be measured is proportional, not to ( -7— ) , that is 

 rfT?2 \an J 



to F 2 . but to ^-. 



7 an 



The preceding three examples, and many others, are dis- 

 cussed in Duhem's Uaimantation par influence — a book 

 which seems to contain the first systematic application of the 

 principles of thermodynamics to magnetism. 



XXXV. The Expansion of Chlorine by Light as applied to 

 the Measurement of the Intensity of Rays of High Ref Tan- 

 gibility. By Dr. A. Richardson, Lecturer on Chemistry, 

 University College, Bristol*. 



[Plates III. & IV.] 



IT has been shown by Budde (Phil. Mag. iv. 1871; Pogg. 

 Ann. Ergbd. vi. 1873) that when chlorine is exposed to 

 the influence of sunlight, an expansion of the gas occurs 

 which is independent of the direct heating-effects due to the 

 light ; the volume to which the gas first expands is maintained 

 during exposure provided that the intensity of the light 

 remains constant, contraction to the original volume taking 

 place when the gas is shaded. He further found that the 

 rays of high refrangibility were influential in promoting this 

 change, no expansion being occasioned by the rays at the red 

 end of the spectrum. The application of this property of 

 chlorine to the measurement of the " actinic " f intensity of 

 light was suggested by Budde many years ago, but no further 

 steps appear to have been taken in this direction. 



Some experiments on which I am at present engaged have 

 rendered it necessary that the actinic intensity of light should 

 be measured during periods of many months together, and it 

 seemed possible that the expansion of chlorine by light might 

 be applied to this purpose. As, however, the researches of 

 Bunsenand Roscoe (Trans. Boy. Soc. 1887, p. 381)led them to 

 the conclusion that no change in volume occurred in chlorine, 

 when exposed to light, other than that due to direct heating- 

 effects, it became necessary to repeat some of Budde's experi- 

 ments so as if possible to decide this point. In order to do this 



* Communicated by the Physical Society : read June 26, 1891. 

 t The term " actinic " is used for brevity to denote rays at the violet 

 end of the spectrum. 



