Sir DAVID BREWSTER on the Colours of Natural Bodies. 543 



The analysis of this experiment is very interesting, but as 

 this is not the place to pursue it, I shall only remark, that I 

 have observed the same phenomenon in various other fluids of 

 different colours, that it occurs almost always in vegetable solu- 

 tions, and almost never in chemical ones, or in coloured glasses ; 

 and that it is a phenomenon of opalescence or imperfect trans- 

 parency. One of the finest examples of it which I have met 

 with may be seen by transmitting a strong pencil of solar light 

 through certain cubes of bluish fluor-spar. The brilliant blue 

 colour of the intromitted pencil is singularly beautiful. 



According to the Newtonian theory of colours, the green of 

 plants is of the same order as the yellow and orange into which 

 it is changed when it withers, in consequence of an increased 

 density, or an enlargement of size in the tinging corpuscles. In 

 order to put this opinion to the test of experiment, I extracted 

 the yellow juice from the brilliant yellow leaves of the common 

 laurel. This fluid becomes of a deep red at great thicknesses. It 

 attacks the spectrum powerfully towards the extremity of the 

 green space, a place where it is not touched by the green fluid. 

 It then absorbs the yellow and violet, leaving a bright green, and 

 converting the blue into violet. At greater thicknesses, the 

 violet disappears, and the absorption advances gradually to the 

 red. 



For the purpose of varying the experiment, I extracted the 

 juice of the leaves of the privet, which become of a deep black 

 violet when they wither, a colour which has not the most remote 

 resemblance to any periodical tint. The fluid was a deep red 

 colour, much deeper than that of the darkest port- wine. It at- 

 tacked the red part of the spectrum near the line B of FRAUN- 

 HOFER, at the same place that the green juice attacked it, leav- 

 ing two red bands, the innermost of which vanished at an in- 

 creased thickness. It then absorbed the violet and blue spaces 

 generally ; and having obliterated the middle of the green space, 

 the absorption advanced to the orange rays at D. 



