J76 M. Macaire Pr'msep on the Colouring of the Leaves, &c. 



blue sweet violet ( Viola odorata) gave, in the same manner, a 

 substance of a fine blue, not unlike the preceding. This sub- 

 stance was rendered green by alkalies, reddened with acids ; was 

 soluble in cold water ; and might be kept in a state of powder, 

 were it wished to preserve the colour of violets. As it might 

 be supposed to be the result of the combination of the red chro- 

 mule with a vegetable alkali, I tried to imitate it by a factitious 

 combination of the same kind. I triturated, with a small quan- 

 tity of vegetable alkali, as quinine, strychnine. Sec. the red chro- 

 mule obtained from leaves of that colour ; and I found that this 

 mixture had become soluble in cold water, had no longer the re- 

 sinous appearance of the red chromule, and assumed a bluish- 

 green tint, as decided as I could have ventured to expect from 

 an experiment imitating the natural processes so widely. This 

 mixture reddened with the acids, and became bluish by means 

 of the alkalies, precisely in the same manner as the vegetable 

 blue tincture would have done. Ammonia in vapour also gives 

 a bluish tint to the red chromule ; but by heat and exposure to 

 the air, the gas evaporates, and the red colour re-appears. 



From these facts, I think it may be concluded, that the blue 

 and violet flowers have, for their colouring principle, the red 

 chromule united to a vegetable alkali. This conclusion I shall 

 try to corroborate by analysis, as soon as the season will allow. 



I had occasion, last spring, to examine different varieties of 

 Aquilegia vulgai'is, unfortunately before engaging in the expe- 

 riments which form the subject of this memoir. This flower, 

 which is naturally blue, easily varies to red, by passing through 

 the intermediate tints. The blue and red flowers, treated se- 

 parately, whether with water or with alcohol, presented tinc- 

 tures, assuredly neutral, and perhaps even alkaline in the first 

 case, and decidedly acid in the second. I even found it to be 

 acetic acid that the red flowers had yielded to the fluids em- 

 ployed. 



From a review of the facts mentioned above, I think there 

 may be deduced the following conclusions : 



1^^, All the coloured parts of vegetables appear to contain 

 a particular substance (the chromule), capable of changing co- 

 lour by slight modifications. 



2ff/^, It is to the fixation of oxygen, and to a sort of acidifica- 

 tion of the chromule, that the autumnal change of the colour of 

 leaves is owing. 



