I believe it may come to be a matter of great commercial importance to discover, at home or abroad, some cheap and easily-procurable substitute for the Roccellas, which are gradually becoming scarce, and consequently valuable in European commerce, having sometimes fetched, in times of scarcity, no less than £1,000 per ton. No plants can be so easily collected and preserved as lichens—requiring merely to be cleaned, dried, pulverised, and packed; and if their bulk be an objection to transport, their whole colorific matter may be collected in the way I have already mentioned. Ascending to the verge of eternal snows, and descending to the ocean level—with a geographical diffusion that is co-extensive with the surface of our earth, it is difficult to say where lichens shall not be found. There are myriads of small rocky islets in the boundless ocean, and there are thousands of miles of barren rocky coast and sterile mountain range in every part of the world, which, though at present unfit to bear any of the higher members of the vegetable kingdom, are yet carpeted and adorned with a rich covering of lichens, and of those very species too, which I have already spoken of as prolific in colorific materials. I sincerely believe, therefore, that a more general attention to the very simple tests just enumerated, would ultimately result in a greatly extended use of the lichens as dye agents. What renders it very probable that efforts in this direction are likely to meet with success is the great similarity of species found all over the world. It has been repeatedly noticed that the European species, which, of course, are best known, differ little from those of North America. Dr. Robert Brown remarked the same fact with regard to New Holland species, and Humboldt also recognised the similarity in natives of the South American Andes. Of a large collection made by Professor Royle, in the Himalayas, Don pronounced almost every one to be identical with European species. From examining the raw vegetable products, sent by different countries to the Great Exhibition of 1851, I am satisfied that, even now, there are many fields open for the establishment of an export trade in Roccellas and other so-called orchella weeds." I there saw specimens of good dye lichens from almost every part of the world, including our own young colonies; and as a single instance of their probable value, I may introduce here the copy of a note appended to a specimen of orchella weed from the island of Socotra, contained in the Indian collection of that exhibition, "abundant, but unknown as an article of use or commerce. Also abundant on the hills around (Aden) and might be made an article of trade." Roccellas from this source are estimated as worth £190 to £380 per ton. I believe that a similar statement might be made with regard to the countless islands of the broad Atlantic and Pacific, which may, at some future period, perhaps not far distant, be found to be rich depots of orchella weeds, just as some of them are, at present, rich fields of guano, and may, as such, become new nuclei of British commerce and enterprise. Even at home, in the immediate vicinity of Edinburgh, or, to restrict our limits still more narrowly, within the compass of Arthur's Seat, there are not a few very good dye-lichens, which require merely to be scraped with an old knife or similar instrument, from the rocks to which they adhere, and subjected to the ammonia process already mentioned. Of twelve specimens thus collected at random one morning, I found no less than three yielded beautiful purple-red colors, apparently as fine as orchil or cudbear, while the others furnished rich and dark tints of brownish-red, brown and olive-green.
Dr. Lindley's communication was illustrated with specimens of coloring matters yielded by various lichens collected in the neighbourhood of Edinburgh, &c.
BARKS FOR TANNING.
Let us now take a brief review of the sources from whence tanning materials may be obtained, which will also enable us to form a fair estimate of the prospect of future supplies. Only one medal was awarded, at the Great Exhibition, for tanning substances, viz., to Messrs. Curtis, Brothers (United Kingdom, No. 126), but honorable mention was made of the following competitors:—One from Tunis, one from Van Diemen's Land, one from New Zealand, one from Belgium, one from the Cape of Good Hope, one from Canada, and one from the United Kingdom.
The substance from which pure tannin is most frequently obtained for chemical purposes is nutgalls, for tannin constitutes above 40 per cent, of their weight. It may be procured also from several other sources, such as oak, horse chestnut, sumach, and cinchona barks, catechu, kino, &c.
The basis of the skins of animals is composed of a substance to which the name of gelatine is given. One of the properties of this substance is, that when combined with tannin, it forms the compound of tannate of gelatine, or leather, a substance which is so useful to mankind. From time immemorial, the substance employed to furnish the tannin to the hides of animals, in order to convert them into leather, has been oak bark. But as the purpose for which oaks are grown is their timber, and not their bark, the supply of oak bark cannot be calculated upon, and this is, perhaps, one of the causes why tanning as an art is in such a backward state.
The consumption of tannin required in the leather manufacture may be estimated from the fact that more than 672,000 cwts. of raw hides were imported in 1851, besides the hides of the cattle, &c., consumed in the United Kingdom. On the Continent and in the United States the consumption of bark for this purpose is also considerable.
The imports of bark for the use of tanners and dyers has amounted yearly to the very large quantity of 380,674 cwt., besides what we obtain at home. Oak bark contains usually the largest proportion of tannin, and according to Davy's experiments eight-and-a-half pounds of oak bark are equivalent for tanning purposes to two-and-a-quarter of galls, three of sumach, seven-and-a-half of Leicester willow, eleven of Spanish chesnut, eighteen of elm, and twenty-one of common willow bark. Tannin obtained from these sources, however, differs materially in some of its characters. The tannin of nutgalls, which is that generally employed for chemical purposes, is sometimes called gallo-tannic acid, to distinguish it from other species.
Notwithstanding the number of different substances which have from time to time been introduced for the use of tanners, it is, nevertheless, pretty generally acknowledged that there is nothing superior, or even equal, to good oak bark, and that all attempts to hurry the process beyond a certain point by the use of concentrated solutions of tan, &c., are for the most part failures, as the manufacture of good leather, to a great extent, depends on the process being conducted in a slow and gradual, but—at the same time—thorough and complete matter.
Oak bark is, however, by no means the only astringent bark well suited to the use of the tanner, and in various parts of the world other similar substances are used with very great success. All these tanning materials, though they may not be considered by the English tanner equal to the best oak bark, are, nevertheless, of great value to him; they may be employed in conjunction with oak bark, or even as a substitute in times of scarcity, or when the price of oak bark is high; in fact the very existence of such substances tends to keep down and equalise the price of bark, and to prevent it from undergoing those great fluctuations in value which would necessarily occur were it the only tanning material available to our manufacture—("Prof. Solly in Jury Reports of Great Exhibition.")