CIIAULES MACINTOSH, F.R.S.

7

“ Essay on the application of the blue colouring matter of Vegetable bodies.

“ In this Essay, Mr. Macintosh advances the opinion that the colouringmatter contained in vegetable substances, is an inherent and separate prin-ciple. He distinguishes animal from vegetable substances by the greaterrapidity with which animal substances run into the putrefactive fermenta-tion, and by the animal substances containing ammonia. As the cruciformplants also contain ammonia, he seems to think that they may be consideredas forming a link between the animal and vegetable kingdoms; and isinclined to think that a somewhat similar position may be assigned to theflesh of young animals, and to tallow, from the readiness with which theyrun into the acetous fermentation ; and from the production of an acid fromtallow by destructive distillation; and then goes on to argue that it ismerely a different proportion and arrangement of the same substances, in■which consists the apparent difference between the animal and vegetablekingdoms.* He alludes to the Indigofera Tinctorea, as the only plantknown at the time, yielding blue fecula, and describes the processes bywhich indigo is obtained from this plant. The violent agitation requisite toseparate the colouring matter from an aqueous solution of this plant, helooks upon as analogous to the process of churning, in separating butterfrom milk. He then narrates some experiments made by him on the Mer-curialis Perennis, or Dog’s Mercury, a native of Scotland , which ripens inthe month of May. By fermenting a solution of this plant in water, heobtained a fine blue; but, contrary to his expectation, by agitating thissolution, after the manner described for obtaining indigo, the blue colourchanged to a dirty brown, the same effect being produced upon evaporatingthe blue solution by heat. When the blue solution was allowed to run intothe putrefactive fermentation, it also changed to a brown. It struck himas a curious fact, that even the mineral acids did not change to red the blueliquid ;f whilst alkalies changed it to green, and a green precipitate followedupon the addition of alum; whilst a solution of bismuth in aqua fortisoccasioned a fine blue precipitate. He next describes the method of dyeingwith indigo, and overthrows the theory of Mons. Hellot, that the bluecolour of indigo-dyed cloths was owing to the adhesion of vitriolated tar-tar to the cloth, seeing that this salt is soluble in water, whilst indigo blueis fixed. He also overthrows Dr. Irvine’s theory, that the gummy particlesof indigo, when changed into resins by the process of dyeing, and becomingthus insoluble in water, remain as fixed colours upon the cloth, by showingthat these resins are all soluble in spirit of wine; and would, by the appli-cation of such spirits, be discharged from indigo-dyed cloth, whilst in realitythey are not. He then argues that the salts used in the indigo vat become

* It is very remarkable that the whole of this hypothesis of Mr. Macintosh’s wasafterwards elucidated and verified by the experiments and discoveries of the French chemists, Chevreul , Braconnet, and Tlienard, in 1813, 1814, and 1815, down tothe discovery of the substances stearin, elain, and margaric and oleic acids, existingin, and being reciprocally obtainable from, animal tallow, and the fixed oils of thevegetable kingdom ; a fact of which chemists, at the date at which Mr. Macintoshwrote, were entirely ignorant. —Ed.

f The test of the blue being vegetable, and which, in this instance, seems tohave failed. —Ed.