I €4. l'ourcroy has made several observations on the effects of oxygen on colouring particles, which enable us to form some opinion respecting the nature of the changes which they undergo. Ills experiments were chiefly made with watery solutions of colouring matter left exposed to the atmosphere, or subjected to a boiling heat. lle observed, that, in consequence of the action of the air, vegetable decoctions formed pellicles, which lost their solubility., and underwent successive changes of colour. lie marked the gradations of colour thus produced, and concluded from his observations, that oxygen entered into the composition of the colouring particles; that when it combined with them, their shade wa, changed, and that the more they received, the more fixed their colour became. Ile therefore inferred, that the best method of obtaining permanent colours for painting. was to choose such as had been exposed to the action of the oxymuriatic acid.
163. It appears, that, in some cases, the absorption of oxygen by the colouring matter produces a change of rolour, in consequence of the oxygen combining with the hydrogen, and thus forming water ; so that the change which takes place is rather to be ascribed to the separation of hydrogen, than to the permanent union of the oxygen with the colouring particles. Berthoilet observed, that the oxymuriatic acid exhibited different phenomena with the colouring matter; that sometimes it discharged their colcur and rendered them white; that most frequently it changed them to a yellow, fawn co lour, brown, or black, according to the intensity of its action; and that, when their colour appeared only dis charged or rendered white, heat, or a length of time, was capable of rendering them yellow, Ile concluded, +hat the effects of the oxymuriatic acid were similar to those of combustion, occasioning a destruction of the hydrogen, which, combining with the oxygen more easily, and at a lower temperature than charcoal does, leaves the latter predominant; so that the natural colour of charcoal is more or less blended with the original co lour. Oxygen has certainly an influence in many other ases on the changes which take place in the colouring particles of vegetables. These particles are formed chiefly in the leaves, flowers, and inner bark of trees ; by degtecs they undergo a slight combustion, either from the action of the atmosphdic air which surrounds .hem, or from that of the air which is carried by a parti cular set of vessels into the internal parts of vegetables. trend: most trt es contain fawn coloured particles, in clining more or less to yellow, red. or brown, which, in consequence of this combustion, grow thick, and are at last thrown out of the vascular fibres of the bark.
166. from what has been stated, it appears, tha changes may bo produced in the colours of bodies, ei ther by the separation of oxygen, or by the union of that principle with one of the elements of the colouring mat ter. In the latter case, which applies chiefly to vegeta ble substances, the oxygen combines with the hydrogen; the attraction of the colouring particles for their basis is thus weakened, and they arc easily carried off by water. This effect takes place more or less rapidly, according to the nature of the colouring particles, or rather, ac cording to the proper ties which they possess in the state of combination into which they have entered. Colouring substances, therefore, resist the action of the air the less they are disposed to unite with oxygen, and thereby suffer, more or less quickly, a smaller or a greater degree of combustion. Light promotes this effect, which, io many' cases, is not produced without its assistance; but the colouring matter, in its separate state, is more liable to suffer this combustion than when united to a substance such as alumine, which may either defend it by its own power of resisting combustion, or, by attract ing it strongly, weaken its action on other substances, which is the chief effect of mordants; and, filially, this last compound acquires still greater durability when it is capable of combining intimately with the stuff.
167. Dr Bancroft has stated several objections to this theory, some of which appear to possess considerable weight, while others are founded upon limited views of the subject. Berthollet has stated, that, in consequence of oxygen combining with the hydrogen of colouring matter, the charcoal becomes predominant, and mani fests its presence by the brownish appearance which the colouring matter exhibits. Dr Bancroft seems to con ceive, that this explanation implied, that charcoal exist ed in vegetable matters in the same state in which it is found in charred wood, and that it was naturally of a black colour ; but this supposition is by no means ne cessary for the truth of the theory It is a certain fact, that in whatever way the carbonization of vegetable mat ter is effected, the charcoal obtained is always of a dark colour : this is the case, whether the vegetable matter be subjected to combustion in close vessels, or placed in a situation (under water, for example) where the volatile matters either make their escape, or unite with the bo dies with which they ate in contact. The charcoal thus obtained, may not have existed in the very same state in the vegetable substance from which it has been procur ed ; and, indeed, there is every reason to conclude, that charcoal is a combination of oxygen and pure carbon; but, at any rate, it is enough for I\1. Berthollet to state. in support of his opinion, that in all cases when the vola tile parts of vegetable matter have made their escape, the carbonaceous part which remains behind is Mick. We conceive, therefore, that Dr Bancroft has done no thing to disprove the correctness of Berthollers opinion, by simply maintaining., that " he should never be con vinced that these matters had naturally contYmed ready formed black charcoal, and that the degradation of the faded or injured colour, resulted from a greater mani festation, and predominance of this charcoal, with its supposed naturally black colour." 168. As a practical inference from his theory, Berthol let has proposed to employ the oxymuriatic acid for de termining the durability of colours : When we wish to examine a colour," says he, we need only' put a pattern of the stuff into oxygenated muriatic acid, along with a pattern of the same colour which has been properly dyed ; the comparative power of resisting its action, shewn by the colours of the two patterns, becomes the measure of their goodness ; but as this liquor has a very strong action on the colouring particles, it must be em ployed in a very dilute state. This proof has also the advantage of shewing us very nearly the shades and changes through which a stuff must pass, when it comes to be acted upon by the air. Still, however," he adds, with much philosophic candour, " were it necessary to examine a colour juridically, I should not venture to rely entirely upon the action of the oxygenated muriatic acid, and would only decide with confidence, from the results afforded by exposure to the air." He afterwards asserts, however, that the oxygenated muriatic acid seems to furnish a scale for the comparison of colours as com pletely as could be desired ; because, when we compare indigo with indigo, the nature of the colouring particles is the same, or nearly the same, and no foreign affinity can interfere, to prevent the action of the oxygenated muriatic acid from determining the proportional quan tity of colouring matter.