402 Sir D. Brewster 07i the Decomposition and Dispersion 



was the alcoholic solution of tiie colouring matter of the leaves 

 of the common laurel. At first its colour is a bright green, 

 afterwards chanfjinji into a fine olive colour : but in all its 

 stages it disperses light of a brilliant blood-red colour, which 

 forms a striking contrast with the transmitted tint. After a 

 long exposure to light, the transmitted tint almost wholly 

 disappears, while the dispersed light retains its red colour*. 

 Another very remarkable example of internal dispersion, 

 pointed out to me by Mr. Schunck, is exhibited in an alkaline 

 or in an alcoholic solution of a resinous powder produced from 

 orcinc by contact with the oxygen of the air. Its colour by 

 transmitted light is reddish brown, and the light which it di- 

 sperses is of an exceedingly rich green colour. 



Since these experiments were made, my attention has been 

 called to two interesting papers by Sir John Herschel in the 

 last part of the Philosophical Transactions ; the one 0)i a case 

 of siqierjicial colour presented by a homogeneous liquid, internally 

 colourless, and the other on the epip)olic [or stipei^cial) dispersion 

 of light ; and as these papers contain results incompatible with 

 those which I had previously published, I found it necessary 

 to resume the investigation of the subject. 



The two papers now referred to are chiefly occupied with a 

 description of the phsenomena of coloured dispersion, as ex- 

 hibited in a diluted solution of sulphate of quinine in weak 

 sulphuric acid. Owing to the solution being nearly colourless 

 by transmitted light, the general phasnomenon is very beau- 

 tiful. The line of bright blue light dispersed by the stratum 

 of fluid immediately beneath the surface of incidence, and 

 about the fiftieth of an inch thick, appears to be confined to 

 that stratum ; and it is in this respect onl}' that the phaeno- 

 menon differs from that which is exhibited by fluor-spar and 

 the vegetable solutions which I have mentioned. 



1. On the Internal Dispersion of Flttor-Spar. 



There are many varieties of fluor-spar in which no disper- 

 sion of the intromitted light takes place. It does not exist in 



* I fliowed this experiment in 1 836, at Lacock Abbey, to Mr. FoxTalbot, 

 and several members of the British Association. At the meeting of the 

 British Association at Manchester, in 1842, a friend handed to me, in the 

 sectional meeting, a " sohition of stramonium in a£ther," which dispersed a 

 bright green light. I described the phainonienon to the meeting, and it is 

 noticed in the Transactions of the Sections, p. 14. Upon making the so- 

 lution myself, I cannot obtain the same tints, either from the stalk or the 

 dried leaves of the plant. The solution of the leaves disperses a brilliant 

 red tint, like that mentioned in the text. The solution put into my hands 

 must therefore have been one of the seeds of stramonium, or of some other 

 substance possessing internal dispersion in a high degree. 



