XV. Contributions to the chemical history of Archil and of Litmus

Among the numerous and complex questions as to the constitution of organic substances, which have latterly attracted the attention of chemists, there is scarcely one possessed of more interest to the manufacturer, as well as to the philosopher, and the elucidation of which might better be expected to lead to improved processes in the arts, or to throw more light on difficult points of abstract theory, than the study of the nature and mode of origin of those remarkable colouring materials which form the basis of the archil and litmus of commerce, and which are obtained from lichens of various genera and species, themselves totally devoid of colour. Although the problem of the origin and nature of these bodies has never been con­templated by chemists in the general point of view, by which alone consequent and satisfactory results could have been arrived at, yet from a very early period in organic chemistry, attention had been directed to isolated portions of it, particularly with regard to litmus, which from its general use as a reagent excited curiosity, and became the subject of frequent, though incomplete examination. Indeed, the nature of litmus appears to have been to many chemists peculiarly obscure, as not withstanding the researches of Fourcroy and Vauquelin, of Tennant, Chevreul, Peretti, Desfosses, and many others, Berzelius declared in the last year that the chemistry of litmus remained yet to be created. Regarding archil, still less knowledge has been obtained. I am not aware of any writer who has occupied himself directly with its examination; and, indeed, it is only incidentally that Heeren mentions, in his admirable memoir of the lichen products, any facts belonging to the substance found in commerce.