316 Proceedings of the Royal Society of Edinburgh. 



author changes to Chromogen. When an acid, added to a vegetable 

 infusion, causes a red colour, and an alkali a yellow or green, it has 

 beenthe universal opinion that both sets of agents act upon one 

 and the same colourable principle. The leading object of this pa- 

 per is to shew that the Chromogen, or colourable principle, is not 

 an individual substance ; and that there are two distinct principles, 

 one which forms the red compound with acids, which he denomi- 

 nates Erythrogen ; and another, which affords a yellow compound 

 with alkalies, which he calls Xanthogen. 



To establish that opinion, Dr Hope made many experiments on 

 the leaves and flowers of plants, with various reagents, principally 

 water, alcohol, acids, and alkalies : and has exhibited the results in 

 the compendious form of tables. The first table presents the result 

 of experiments on the leaves of many plants ; and the general result 

 from them, in regard to the special object of inquiry, is, that in ad- 

 dition to the green Chromule, denominated Chlorophyle by many 

 wi-iters, they all contain Xanthogen, and that none of them, except- 

 ing those which have some tint different from the green, contain 

 Erythrogen. 



The second table exhibits the result of the action of the reagents 

 upon white flowers, all of which, to the number of about thirty, 

 gave proofs of their containing Xanthogen, but no Erythrogen nor 

 tinted Chromule of any kind. 



The third table displays the results with yellow flowers, from 

 which the general inferences are, that the yellow Chromule varies 

 in its nature in different flowers; that all those subjected to expe- 

 riment contained Xanthogen, and none of them Erythrogen. 



The fourth table exhibits the experiments with red flowers, and 

 affords the general conclusions, that while the red chromule shews 

 considerable variety of character, red flowers contain both Xantho- 

 gen and Erythrogen in abundance. 



The fifth table exhibits the results with twenty blue flowers, and 

 presented the general observations, that the blue Chromule varies in 

 its character in different blossoms, particularly in shewing very dif- 

 ferent degrees of solubility in water and alcohol, and in some pro- 

 ducing coloured, and in others colourless, solutions in both men- 

 strua ; and that they contain both the colourable principles of Xan- 

 thogen and Erythrogen. 



The sixth table relates to ten orange flowers, which equally 

 shews that the orange Chromule differs much in one plant from 

 another, and that they contain both colourable principles. 



The seventh table relates to twenty purple flowers, and afforded 

 the same conclusions as the preceding. 



The eighth table exhibits the experiments made upon the tinted 

 Chromule found in other parts of plants, beside the corolla of the 

 flowers, e. g, the calyx, bractea, the coloured leaves of plants, fruits, 

 and surface of the roots, all which comported themselves as the* 

 corresponding coloured Chromules of the flowers do. 



Litmus presented a solitary example, but a very interesting one_ 

 in this inquiry, of a substance abounding largely in Erythi-ogen, but 

 containing no Xanthogen. 



