(See Chemistry-Index in this Suppl.), is the principal agent in the new process of bleaching (see Bleaching, Suppl.); but, till very lately, at least, if not even at present, the bleachers were in the practice of adding some alkali to the acid, notwithstanding the strong objections which M. Bertholet made to that addition, and notwithstanding the proofs urged by Mr. Rupp, that it increases the expense of bleaching about 40 per cent. The chief reason for persisting in a practice to which such objections were urged was, that the addition of the alkali deprives the liquor of its suffocating effects without destroying Oxy-Muriatic Acid's bleaching powers. Mr. Rupp, however, has contrived the following apparatus, in which may be safely used the pure oxy muriatic acid simply dissolved in water, which is at once its cheapest and best vehicle.

Figure 1. (Plate XLII.) is a section of the apparatus. It consists of an oblong deal cistern ABCD, made water-tight. A rib EE of ash or beech wood is firmly fixed to the middle of the bottom CD, being mortised into the ends of the cistern. This rib is provided with holes at FF, in which two perpendicular axes are to turn. The lid AB has a rim GG, which sinks and fits into the cistern. Two tubes HH are fixed into the lid, their centres being perpendicularly over the centres of the sockets FF when the lid is upon the cistern. At I, is a tube by which the liquor is introduced into the apparatus. As it is necessary that the space within the rim GG be air tight, its joints to the lid, and the joints of the tubes, must be very close; and, if necessary, secured with pitch. Two perpendicular axes KL, made of ash or beech wood, pass through the tubes HH, and rest in the sockets FF. A piece of strong canvas M is sewed very tight round the axis K, one end of it projecting from the axis. The other axis is provided with a similar piece of canvas. N are pieces of cloth rolled upon the axis L. Two plain pulleys OO are fixed to the axes, in order to prevent the cloth from slipping down. The shafts are turned by a moveable handle P. Q is a moveable pulley, round which passes the cord R. This cord, which is fastened on the opposite side of the lid (see fig. 2.), and passes over the small pulley S, produces friction by means of the weight T. By the spigot and faucet V, the liquor is let off when exhausted.

The dimensions of this apparatus are calculated for the purpose of bleaching twelve or fifteen pieces of calicoes, or any other stuffs of equal breadth and substance. When the goods are ready for bleaching, the axis L is placed on a frame in an horizontal position, and one of the pieces N being fastened to the canvas M by means of wooden skewers, in the manner represented in fig. 1, it is rolled upon the axis by turning it with the handle P. This operation must be performed by two persons; the one turning the axis and the other directing the piece, which must be rolled on very tight and very even. When the first piece is on the axis, the next piece is fastened to the end of it by skewers, and wound on in the same manner as the first. The same method is pursued till all the pieces are wound upon the axis. The end of the last piece is then fastened to the canvas of the axis K. Both axes are afterwards placed into the cistern, with their ends in the sockets FF, and the lid is put on the cistern by passing the axes through the tubes HH. The handle P is put upon the empty axis, and the pulley Q upon the axis on which the cloth is rolled, and the cord R, with the weight T, is put round it and over the pulley S. The use of the friction, produced by this weight, is to make the cloth wind tight upon the other axis. But as the effect of the weight will increase as one cylinder increases and the other lessens, Mr. Rupp recommends that three or four weights be suspended on the cord, which may be taken off gradually as the person who works the machine may find it convenient. As the weights hang in open hooks, which are fastened to OXY

Musithe cord, it will be little or no trouble to put them on atic Acid, and to remove them.

Things being thus disposed, the bleaching liquor is to be transferred from the vessels in which it has been prepared into the apparatus, by a moveable tube passing through the tube I, and descending to the bottom of the cistern. This tube being connected with the vessels, by means of leaden or wooden pipes provided with cocks, hardly any vapours will escape in the transfer. When the apparatus is filled up to the line a, the moveable tube is to be withdrawn, and the tube I closed. As the liquor rises above the edge of the rim G, and above the tubes HH, it is evident that no evaporation can take place, except where the rim does not apply closely to the sides of the box; which will, however, form a very trifling surface if the carpenter's work be decently done. The cloth is now to be wound from the axis L upon the axis K, by turning this; and when this is accomplished, the handle P and pulley Q are to be changed, and the cloth is to be wound back upon the axis L. This operation is, of course, to be repeated as often as necessary. It is plain, that by this process of winding the cloth from one axis upon the other, every part of it is exposed, in the most complete manner, to the action of the liquor in which it is immersed. It will be necessary to turn, at first, very briskly, not only because the liquor is then the strongest, but also because it requires a number of revolutions, when the axis is bare, to move a certain length of cloth in a given time; though this may be performed by a single revolution when the axis is filled. Experience must teach how long the goods are to be worked; nor can any rule be given respecting the quantity and strength of the liquor, in order to bleach a certain number of pieces. An intelligent workman will soon attain a sufficient knowledge of these points. It is hardly necessary to observe, that, if the liquor should retain any strength after a set of pieces are bleached with it, it may again be employed for another set.

With a few alterations, this apparatus might be made applicable to the bleaching of yarn. If, for instance, the pulley O were removed from the end of the axis K, and fixed immediately under the tube H,—if it were perforated in all directions, and tapes or strings passed through the holes, skeins of yarn might be tied to these tapes underneath the pulley, so as to hang down towards the bottom of the box. The apparatus being afterwards filled with bleaching liquor, and the axis turned, the motion would cause every thread to be acted upon by the liquor. Several axes might thus be turned in the same box, and being connected with each other by pulleys, they might all be worked by one person at the same time; and as all would turn the same way and with the same speed, the skeins could not possibly entangle each other.

P.

Encaustic painting is an art of very high antiquity, which, after being lost for many ages, was restored, as is commonly believed, by the celebrated Count Caylus, whose method was greatly improved, first by Mr. Joshua Colebrooke, and afterwards by Miss Greenland, who brought the rudiments of her knowledge from Italy (See Encaustic, Encycl.). In that country encaustic painting had employed the attention of various artists and men of learning, such as Requeno, Longus, and Astori, &c.; but the best account of it that has fallen under our notice, is in that valuable miscellany called the Philosophical Magazine, taken from a work of Giov. Fabbroni, published at Rome in the year 1797.

According to this author, "the knowledge and use of encaustic painting is certainly older than the time of the Greeks and the Romans, to whom the learned Requeno seems to assign the exclusive possession of this art; because the Egyptians, who, with the Etruscans, were the parents of the greater part of the inventions known among mankind, and from whom the Greeks learned so much, were acquainted with and employed encaustic painting in the ancient ages of their greatness and splendour, as is proved by the valuable fragments of the bandages and coverings of some mummies which he had examined. No oil-painting (he says), of only two or three hundred years old, exhibits a white paint which has kept so well as that seen on these fragments; and this circumstance sufficiently proves the superiority of the encaustic method over the common oil-painting, which, notwithstanding the general opinion, cannot, he thinks, have been unknown to the ancients.

"It is impossible (says he) that in Egypt and Phoenicia, where so much use was made of flax, the oil procured in abundance from that plant should have been unknown. Those who have kept oil, or who have spilt any of it, whether nut or linseed oil, must have remarked that it possesses the property of soon drying by the effects of the atmosphere; and therefore it may be easily believed that mankind must soon have conceived the idea of employing it, particularly for ships, which, as Herodotus says, were painted with red ochre in the earliest periods, and adorned with figures and ornaments. The use of oil afforded painting a much simpler and easier method than that of wax; it must therefore have been first adopted, and the transition from oil to wax must be considered as a step towards bringing the art to perfection; because encaustic painting is not exposed to the irremediable inconveniences that arise in oil-painting, the value of which we extolled through ignorance, and praised as a new invention.

"Oil in general, and in particular drying oil which the painters use, has naturally a strong inclination to combine itself with the vital air or oxygen of the atmosphere; and by imbibing oxygen it becomes dry, and assumes the character of resin; but the colour then becomes darker, as is the case with transparent turpentine, which gradually becomes a black pitch." According to the new and more accurate method of decomposing bodies, oil consists principally of hydrogen and carbon. By coming into contact with the atmosphere, and absorbing its oxygen and light, it undergoes a slow and imperceptible combustion, which is not essentially different from the speedy and violent one which it would undergo in the common mode of burning. It first passes, by inhaling oxygen, into the state of a more or less dark resin; loses gradually its essential hydrogen, which makes a new combination, and afterwards the oxygen itself which has attracted the carbon; and at length leaves behind a thin layer of actual carbon, which in the end becomes black in the course of time, and considerably obscures the oil painting. By a continuance of the before-mentioned slow combustion, the carbon itself, as it were, burns also; if it be strongly acted upon by the light, it attracts the oxygen of the atmosphere, and again brings forward the carbolic acid or fixed air, which gradually flies off. By this, which I may call the second degree of combustion, the painting must become dusty and friable, like crayon painting.

Hence it appears (says our author) that one can hope only for a transient or deceitful effect from the refreshing of oil-paintings with oil; because the harmony of the tones, which the painter establishes as suited for the moment, does not proceed with equal steps, and cannot preserve itself in the like measure for the course of a few years, as each tint, as they say, ought to increase, or, to speak more properly, to burn in proportion to its antiquity. It thence follows, that mere washing may be prejudicial to an old painting; and that the method of refreshing paintings, as it is called, by dabbing over the surface, from time to time, with new drying oil, is highly prejudicial and ill calculated for the intended purpose, since the oil when it becomes dry contracts in its whole surface, carries with it the paint under it, and occasions cracks in the painting. New oil of this kind gives occasion to mineral paints to be restored; but covers the picture with a new coat of resin, and then of carbon, which arises from the gradual combustion, and always causes more blackness, and the decay of the painting which one wishes to preserve.

Wax, on the other hand, undergoes a change which is very different from that of drying oil. The wax, instead of becoming black by the contact of the atmosphere, increases in whiteness, and, according to its natural quality, is not decomposed in the air, and it does not strongly attract the oxygen of the calces or metallic oxides which are commonly used in painting. Moreover, the so-called earths, which are in themselves white, and are never variable either by the presence or absence of oxygen, cannot be employed in oil-painting, because that fluid makes them almost transparent, and causes them to remain as it were without body, and not to produce the wished-for effect. That beautiful white, which may be observed on the before-mentioned Egyptian encaustic, is nothing else than a simple earth, and according to our author's chemical experiments, a chalk which is also unalterable.

That the ancients were once acquainted with the use of oil-painting, and neglected it on account of the great superiority of the encaustic method, our author thinks farther evident from the different accounts which we have of the ancient paintings. Thus Petronius praises the fresh appearance which the valuable works of Zeuxis and Apelles had, even in his time; but Cicero, on the other hand, speaks of the paintings of the ancients having suffered from blackness. The former speaks of wax-painting, and the latter certainly alludes to paintings in oil. It is well known that paintings with wet chalks or water colours do not become black by age, and that this is the case also with encaustic. Of this any one may be convinced, not only by the expressions of the above quoted authors, but by one's own eyes on surveying the Egyptian fragment alluded to. Galland proves, on various grounds, that a painting was made with oil so early as the reign of Marcus Aurelius; and if no specimens of that period have reached us, this is perhaps to be ascribed to the frail and perishable nature of this species of painting.

Sign. Fabbro, after some farther observations, calculated to prove that metallic oxides or calces could not have been employed as pigments on such mummies as still retain their colours fresh, proceeds thus: "Those who are acquainted with the accuracy and certainty of the method not long since introduced into chemical operations, will be convinced, that in 24 grains of the encaustic painting, which I ventured to detach from the above-mentioned Egyptian fragment, in order to subject it to examination, the mixture of an hundredth part of a foreign substance would have been discovered with the greatest certainty; that the resin of Requeno must undoubtedly have been perceptible to me, and that the alkali of Bachelier and Lorgna could not have escaped the counteracting medium. But in this Egyptian encaustic I found nothing except very pure wax, though I varied my analysis in every known method. I must therefore conclude, that modern learned writers, at least in respect to this Egyptian mode of painting, were as far from the truth as the accounts of ancient authors appear to me precise and satisfactory; and that the encaustic with which formerly the fore parts of ships and the walls of houses and temples were painted, was something different from soap or resinous crayons.

I am well aware that it will be asked, In what manner can wax at present be rendered sufficiently liquid for the strokes of the pencil, if it be not converted into powder or soap? This question, in my opinion, can be fully answered from the words of an ancient author, and, in the next place, by experience.

Vitruvius in particular, book vii, chap. ix, expresses himself in the following clear manner:

"Those (says he) who wish to retain cinnabar on walls, cover it, when it has been well laid on and dried, with Punic wax diluted in a little oil (let this be well remarked); and after they have spread out the wax with a hair brush, they heat the wall by means of a brazier filled with burning coals (hence it is called encaustic painting), and then make it smooth and level by rubbing it with wax tapers and clean cloths, as is done when marble statues are covered with wax. The effect of this wax crust is, that the colour is not destroyed by the light of the sun or the moon (a)."

(a) The reader will find the original of this passage, with a translation somewhat different, in the article Encaustic, Encycl. It here appears, that the Romans, who copied the Grecian process, which the latter borrowed from the Egyptians, mixed the wax with an oil to make it pliable under the brush; but no mastic, alkali, or honey, as has been ingeniously imagined, and which some have thought might be employed with success. The difficulty now will be confined to point out in what manner this oil was employed. It does not appear that they used those fat oils which are commonly called drying oils; because they could have employed these as we do, without the addition of wax, which, in such a case, would have been entirely superfluous. Fat oils which do not dry would not have been proper for that purpose, as they would have kept the wax continually in the state of a soft pomade or varnish. Besides, my experiments (continues the author) would without doubt have shewn me the existence of any oily matter.

With regard to essential or volatile oils, a knowledge of them is not allowed to the ancients, as the invention of distilling is not older than the eighth or ninth century, and therefore falls in with the period of Geber or Avicenna. Yet it is certain, that, in order to use wax in their encaustic painting, they must have combined it with an ethereal volatile oil, of which no traces should afterwards remain; because this was necessary for the solidity of the work, and because no oil was found in the fragment that was examined. But naphtha is such an oil, much lighter (says our author) than ether of vitriol itself. It is exceedingly volatile, and evaporates without leaving a trace of it behind. On this account it is used when signatures and manuscripts are to be copied; because the paper, which is moistened by it, and so rendered transparent, quickly becomes white and opaque as before by the complete evaporation of the naphtha. That the Assyrians, Chaldeans, and Persians, were well acquainted with the properties of naphtha is known to every scholar; and hence our author thinks it highly probable that it was used by those nations to render wax fit for painting. It appears to me (says he) that the Greeks, as was the case with many other things, learned encaustic from the Egyptians, who probably derived it from the Assyrians or Chaldeans; and if so, we have discovered the real mixture used for ancient encaustic painting.

To put the matter, however, beyond a doubt, Sign. Fabbroni prepared, for an eminent Saxon painter, a solution of Venetian wax in highly purified naphtha, desiring him to mix up with it the colours necessary for a painting. The artist complied; and both he and our author were astonished, as well as all their friends, at the high tone which the colours assumed, and the agreeable lustre which the painting afterwards acquired when it had been rubbed over with a soft cloth. A similar solution of wax was made for another artist, in which the spirit of turpentine was used instead of naphtha with equal success. Our author therefore concludes, we think with reason, that if he has not discovered the real composition employed by the ancients in their encaustic paintings, he has at least approached much nearer to that discovery than any of his predecessors who have employed their learned labours in the same field of investigation.