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XX. Abnormal Polarization and Colour of Light scattered 

 by Small Absorbing Particles. By R. W. W(5t>J> *. 



IN the course of some experiments on the fluorescence of 

 iodine vapour, I observed on one occasion the precipita- 

 tion of what appeared to be an iodine fog in one of the glass 

 bulbs. This fog scattered powerfully light of a deep red 

 colour, and on examining it with a nicol prism I was astonished 

 to find that it was plane polarized in a direction at right 

 angles to that which is usually observed in the case of light 

 scattered by small particles. When a powerful beam of light 

 was sent through the bulb in a horizontal direction, the 

 scattered light came off at right angles, with its direction of 

 vibration (electric vector) horizontal instead of vertical. If 

 the light was polarized before it entered the bulb, the light 

 was scattered laterally in the directions of the vibration in the 

 incident light. The phenomenon was first observed in a bulb 

 containing a small flake of iodine, and air at atmospheric 

 pressure, cooled by immersion in a beaker of water to which 

 ice had been added. On attempting to repeat the experiment 

 I met with failure, and had about given up the attempt, when 

 the red fog suddenly appeared again in a bulb which I was 

 exhausting with a mercury pump. In this case the fog was 

 doubtless caused by sudden expansion. This method failed 

 repeatedly in subsequent experiments. I finally found a 

 method which gave the fog with tolerable certainty, namely, 

 to precipitate the iodine upon a smoke-cloud already existing 

 in the bulb. If iodine is vaporized in a bulb and then 

 cooled, it crystallizes either upon the walls, or in the form of 

 floating spicutae, too large to show the scattering effects. If, 

 however, the bulb is filled with a light smoke and the iodine 

 vaporized, on cooling each smoke particle forms a nucleus 

 upon which the iodine crystallizes. The smoke which appeared 

 best was that from hot sealing-wax. Tobacco-smoke did not 

 work well. At first I obtained the fogs bv sucking a little 

 of the vapour given off from a blazing stick of sealing-wax 

 which had just been blown out, introducing a small flake of 

 iodine, and warming it with a flame. Some beautiful red 

 fogs were obtained in this way, but quite as often the scattered 

 light was similar to that obtained from the smoke alone. 

 The best plan appears to be to introduce a bit of sealing- 

 wax the size of a pin-head, and a similar speck of iodine, into 

 a small glass bulb, and then heat them both with a very small 



* Communicated by tho Author. 



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