Repulsion resulting from Radiation. 149 



in the paper graphically as a curve, the maximum being in the 

 ultra-red, and the minimum in the ultra-violet. Taking the 

 maximum at 100, the following are the mechanical values of the 

 different colours of the spectrum : — 



Ultra-red 100 



Extreme red 85 



Eed 73 



Orange 66 



Yellow 57 



Green 41 



Blue 22 



Indigo 8| 



Violet 6 



Ultra-violet 5 



A comparison of these figures with those usually given in text- 

 books to represent the distribution of heat in the spectrum is a 

 sufficient proof that the mechanical action of radiation is as much 

 a function of the luminous rays as it is of the dark heat-rays. 



The author discusses the question, " Is the effect due to heat 

 or to light ?" There is no real difference between heat and. light ; 

 all we can take account of is difference of wave-length ; and a ray 

 of a definite refrangibility cannot be split up into two rays, one 

 being heat and one light. Take, for instance, a ray of definite 

 refrangibility in the red. Falling on a thermometer it shows the 

 action of heat ; on a thermopile it produces an electric current ; to 

 the eye it appears as fight and colour ; on a photographic plate it 

 causes chemical action ; and on the suspended pith it causes 

 motion. But all these actions are inseparable attributes of the 

 ray of that particular wave-length, and are not evidence of separate 

 identities. 



The author enters into some theoretical explanations of the 

 action of the different parts of the spectrum ; but these cannot well 

 be given in abstract. 



An experiment is described by which sunlight was filtered through 

 alum, glass, and water screens, so as to cut off the whole of the 

 ultra-red or dark-heat rays. The ray of light which w T as thus 

 freed from dark heat was allowed to fall on the pith surface of the 

 torsion-apparatus, when it produced a deflection of 105°. On in- 

 terposing a solution of iodine in disulphide of carbon the deflection 

 fell to 2°, showing that the previous action was almost entirely 

 due to light. With a candle tried under the same circumstances, 

 the light filtered from dark heat produced a deflection of 37°, 

 which was reduced to 5° by interposing the opaque solution of 

 iodine. 



In order to obtain comparative results among disks of pith 

 coated with lampblack and with other substances, a torsion-appara- 

 tus was constructed in which two or more disks could be exposed 

 one after the other to a standard light. One disk always being 

 lampblacked pith, the other disks could be changed so as to get 

 comparisons of action. If the action of radiation from a candle 



