DYEING.

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may he restored by taking away this acid, e. g. by chalk and am-moniac, or volatile alkali. But this is not the case with respect tothe action of air and light. The action of the air on colouringmatter is derived principally from the oxygen contained in theatmosphere. M. Berthollet observes, that the air combining withthem, renders them weaker and paler, and bv degrees occasions aslight combustion, by means of which the hydrogen which enteredinto their composition is destroyed ; they change to a yellow, red,or fawn colour; their attraction for the stuff seems to diminish;they separate from it, and are carried off by' water: all these ef-fects vary, and take place more or less readily, and more or lesscompletely, according to the nature of the colouring particles; orrather, from the nature of the properties which they possess, in thestate of combination into which they have gone. The alterationswhich occur in the colours, produced the union of the colouringparticles with metallic oxides, are effects compounded of thechange which takes place in the colouring particles, and of thatwhich is undergone by the metallic oxide. The light of the sunconsiderably accelerates the distinction of colours. It ought,therefore, if this theory be well founded, to favour the combina-tion of oxygen, and the combustion thereby induced. M. Sen-licbier, who has given many interesting observations on the effectsof light on different substances, and particularly on their colours,attributes these effects to a direct combination of lis>lit with thesubstances. And. the effects of light on the colour of wood havelong ago been noticed ; it preserves its natural appearance whilekept iii the dark, but when ever it is exposed to the light, it be-comes yellow, brown, or of other shades. *M. Sennebier liasremarked the varieties which occur in this particular, in differentkinds of wood, and lias fomid that the changes are proportioned tothe brightness of the light, and that they take place even underwater, but that wetted wood underwent these changes less quicklythan that which was dry; that several folds of ribband were re-quired to defend the wood completely, that a single leaf of blackpaper was sullicient, but that when paper of any other colour wassubstituted, the change was not prevented; a single covering ofwhite paper was insufficient, but two intercepted the action of therays of light. M. Sennebier has also .extended his experimentsto a great number of vegetable substances, in a manner that mayserve to illustrate different phenomena of vegetation. If a wellmade solution of the green parts of vegetables in alcohol, whichhas a fine green colour, be exposed to the light of the sun, it verysoon acquires an olive hue, and loses its colour in a few minutes.If the light be weak, the effect is much more slow ; and in perfectdarkness, the colour remains without alteration, or if any changedoes take place, it requires a great length of time. An alkali re-stores the green colour; but if the change of colour in the liquorlias been completed, the alkali has no effect. No change of colourtakes place in azotic gas, nor in a bottle which is exactly full. A bot-tle half full of this green solution was inverted over mercury, 'byM. Berthollet, and exposed to the light of the sun ; wlv-n the colourwas discharged, the memiry was found to have risen in tiie bottle,and consequently oxygen bad been absorbed, having united withthe colouring matter. Light , therefore, acts by favouring the ab-sorption of oxygen, and the combustion of the colouring matter;at ffrst, the marks of combustion are not evident; the liquor re-tains only a slight yellow tinge; but by the assistance of beat, thecombustion is completed, the liquor becomes brown, and leaves ablack risiduum. If the vessel which holds the liquor contain nooxygenous gas, the light has no effect on the colouring matter;azotic gas in this situation does not suffer any diminution. Theobservation, that ribbands, or a single leaf of white paper, do notprevent the action of light , deserves attention, as it shews thatlight can pass through coverings which appear to be opake, andexert its force at some way within. Beecaria and Sennebier havecompared the affects of light on ribbands of various colours ; butthe differences lliey have observed are rather to be attributed tothe nature of the colouring matters than to the colours; for a rib-band dved with Brasil wood will lose its colour much sooner thanone dyed with cochineal, though the shade should be exactly thesame in each. On the whole, it is evident that colouring sub-stances resist the action of the air more or less, according as theyare more or less disposed to unite with oxygen, and thereby tosuffer more or less quickly a smaller or greater degree of combus-tion. Light favours this effect, which in many cases is not pro-duced without its assistance ; but the colouring matter, in its sepa-VOL. II. -NO. t>9.

rate state, is much more prone to this combustion than whenunited to a substance, such as alumine, which may either defend itby its own power of resisting combustion, or by attracting itstrongly, weaken its action on other substances, which is the chiefeffect of mordants. This last compound acquires still greater du-rability, when it is capable of combining intimately with the stuffupon which it is deposited. Thus the colouring matter of cochi-neal dissolves easily in water, and its colour is quickly changed kythe air; but when united to the oxide of tin, it becomes muchbrighter, and almost insoluble in water, though it is still easilyaffected by (lie air, and by oxygenated muriatic acid; it resists theaction of these better, however, when it lias formed a triple com-pound with a woollen stuff. But still it must not be inferred, thatall yellow colours are owing to the carbonic part of the colouringsubstance ; very different compounds are capable of producing thesame colour; thus, indigo is very different from the blue of our(lowers, from that of o.xiae of copper, and from that of Prussianblue .

Of Astringents.

Astringents deserve particular attention, not only from theirgreat use in dyeing, hulas possessing a property common to manyvegetables. Our ideas respecting this property have been ex-tremely vague; some slight resemblance in taste has frequentlybeen the only circumstance attended to, and, under the name ofastringents, alum, as well as many vegetables of very differentproperties, have been confounded together in the arts, as well asin medicine. Almost every substance, which renders a solutionof iron black, lias been considered as astringent. This effect hasbeen supposed to lie owing to one identical principle, existing inall the substances which produce it, ami on which the title astrin-gent has been bestowed. It is still considered as a particular acid,to which the name of gallic acid is given, from gall. ,, in which itis found to be predominant. The gall-nut is an excrescencefound on the young branches of the oak, and produced by thepuncture of an insect. Different kinds of the gall-nut are metwith, sone inclining to white, yellow, green, brown, or red ; othersash-coloured or blackish. They also differ greatly in magnitude,and are either round or irregular, heavy or light, smooth or cover-ed with protuberances. Those which are small, blackish, knotted,and heavy, are the best; and are known by the name of Aleppo galls. These astringent substances are almost totally soluble inwater by long ebullition. Sixteen drams affi-rded Neumann four-teen of extract; from the remaining tw o drams, only lour grainscould be abstracted by alcohol. And the same quantity treatedfirst with alcohol, and then with water, afforded twelve drams andtwoscru pules of spirit nous extract,and four scruples of wateryextract;the residuum weighed half a scruple more than the preceding ex-periment. In the spirituous extract the taste is more strong andI disagreeable than in the watery extract. If the astringent pro-perty were owing to an individual principle, which was always thesame, distributee! in different vegetables, the precipitates obtainedby their means, from a solution of iron, would constantly form thesame compounds, and exhibit the same appearances and properties;but the prec ipitate produced by galls is of a blackish blue ; thatby logwood has a different shade of blue ; that by oak is of a fawncolour, or blackUh brown; that by quinquina a blackish green. Theyfall down with different attendant circumstances, and when fixedon stuff's, are discharged by alum and tartar, some much moreeasily than others; and probably, by multiplying experiments,many other remarkable differences may be discovered in the pro-perties of these different precipitates. Astringents form with irondiff erent species of compounds, and consequently do not derivetheir properties from one principle ; but there must be a propertycommon to different substances, to enable them to act in an uni-form manner on solutions of iron, and to produce precipitatesmore or less black, and thus appearing of the same nature, andthis common propei ty is railed the astringent principle. But,notwithstanding chemists have considered the astringent principleas always the same, exp“rience has taught us, that all astringentsubstances were not equally proper for producing a beautiful anddurable black ; it is of importance to determine which 01 themmay be employed with the greatest success: it is, however, vervdifficult to make comparative experiments on this subject v. itnperfect accuracy, because some substauers require much longerboiling than others to extract their astringeiicv ; because a uilter-3 1 ence