Since the article Bleaching in the Encyclopedia was written, very great improvements have been introduced into the art. Of these improvements we shall proceed to give an account.

Mr Scheele of Sweden discovered the oxy-muriatic Discovery acid, or dephlogisticated muriatic acid, as he called it, of the oxy-muriatic acid, about the year 1774, and soon after observed its effects on vegetable colours. His method of procuring it was as follows: In a sand-bath is to be placed a glass retort, in which muriatic acid has been poured upon manganese; to this small receivers are to be adapted capable of containing about twelve ounces each, into which is to be poured about two drachms of water, without any other tube than a slip of blotting-paper about the neck of the retort. In about a quarter of an hour a yellow air is perceived in the receiver, which is to be taken off. If the paper has been properly applied, the air rushes out forcibly; the receiver must be quickly stopped, and another applied. Thus many receivers may be filled with the dephlogisticated muriatic acid; but it is necessary to place the retort in such a manner that the drops which rise into its neck may be able to fall back. The water serves to retain the vapours of the acid. "I use (says he) many receivers, that I may not be obliged to repeat a similar distillation for every experiment. It is not proper to employ large ones, because every time they are opened, a great part of the acid is dissipated in the air. What I submitted to examination with this dephlogisticated muriatic acid was placed in the neck of the receiver, which I had flopped. The cork was turned yellow, as by aquafortis. Paper tinged with turpentine became almost white; all red, blue, and yellow flowers, as also green plants, turned yellow in a short time, and the water in the receiver was chan- bled into a pure but weak muriatic acid. Neither alkalis nor acids were able to restore the colours of the flowers, or of the plants."

M. Berthollet, in 1785, proved that this acid was composed of muriatic acid combined with oxygen; and that when it had deprived vegetable matters of their colour, it was reduced to the state of common muriatic acid; that is, it had lost the oxygen with which it was united. This oxygen had combined with the colouring particles of the vegetable matter, and had rendered them colourless. After making these observations, it occurred to him that the oxy-muriatic acid might produce the same effect upon those particles which give colour to thread and cloth, and which it is the object of bleaching to destroy. "At first (says he) I made use of water highly impregnated with this acid; and I renewed it when it was exhausted, until the thread or cloth appeared white; but I soon perceived that they were considerably weakened, and that they were entirely losing their solidity. I then weakened the liquor a little, and I succeeded in bleaching cloth without damaging it. But it speedily became yellow by keeping, especially if it was warmed, or passed through an alkaline ley. I reflected upon the circumstances of common bleaching, and I endeavoured to imitate its process, because I thought the oxygenated muriatic acid might act in the same manner as the exposition of the cloth in the meadows, which alone does not suffice, but which appears only to dispose the colouring parts of the cloth to be dissolved by the alkali of the ley. I examined dew, not only that which falls from the atmosphere, but also that which comes from the nocturnal transpiration of plants; and I observed that both of them were impregnated with oxygen, sufficiently to destroy the colour of paper slightly tinged with turpentine.

"I therefore employed leys, and the action of oxygenated muriatic acid, alternately, and I then obtained a permanent white; and as, at the finishing of the common bleaching, the cloth is passed through four milk, or through sulphuric acid diluted with a very large quantity of water, I also tried passing the cloth through a very dilute solution of sulphuric acid, and I observed that the white was thereby rendered more clear. As soon as I made use of the leys immediately, I found that it was not necessary to employ a concentrated liquor, or to let the cloth, at every immersion, remain long therein: by this I avoided two inconveniences, which would have rendered this process impossible to be practised in the large way. The first is the suffocating odour of the liquor, which it would be very inconvenient, and even dangerous, to respire for any length of time, and which has discouraged many persons who tried to use it; the second is, the danger of weakening the cloth. I now also left off mixing any alkali with the oxygenated muriatic acid, as I had practised in the greatest part of my first trials.

"This is nearly the state in which my experiments were, when I made some trials in the presence of the celebrated Mr Watt. A single view sufficed for a philosopher whose genius has been exercised for long upon the arts. In a short time Mr Watt wrote to me from England, that even in the first operation he had bleached five hundred pieces of cloth at Mr Grigor's, who has a large bleaching-ground at Glasgow, and who continues to make use of the new process. In the mean

Suppl. Vol. I. Part I. Bleaching weakened still suffice for the cotton, although they have scarcely any action upon hemp or flax.

"Thread, in the common way of bleaching, is attended with a far greater number of difficulties than cloth; because of the immense number of surfaces which it is necessary to present successively to the action of the atmosphere. Some part of these difficulties occur in bleaching with the oxygenated muriatic acid; nevertheless, in the end, it is more advantageous with respect to thread than with respect to cloth. Mr Weller has formed at Lille, with two partners, an establishment for bleaching thread, with great success, and he has already begun some others. He has found that ten or twelve leys, and as many immersions, are required for some sorts of thread; and, that the thread may be surrounded with the liquor, it is necessary to place it, quite loosely, in a basket, which permits the liquor to penetrate to all its surfaces; when the liquor is much weakened, it is still fit to be used for the bleaching of cotton.

"I had, in the beginning of my experiments, tried whether the vapour would not be preferable to the oxygenated muriatic acid in a liquid state, and I observed that it bleached with greater quickness; but, whatever precautions I employed, it appeared to me that a considerable loss of it took place; that those parts of the cloths which were the most exposed to it were subject to be weakened; and that it was more difficult to obtain an equal whiteness throughout.

"To prevent all the accidents which may result from the liquor acting with too great power, it is important to have a means of measuring its force. M. Decroix thought of using, for that purpose, a solution of indigo in sulphuric acid. He takes one part of indigo, reduced into fine powder, and eighteen parts of concentrated sulphuric acid; this mixture is put into a matras, which is kept, during some hours, in a water-bath; when the solution is finished, it is diluted with a thousand parts of water. To try the power of the oxygenated muriatic acid, one measure of this solution is put into a graduated glass tube, and some of the liquor is gradually added to it, until the colour of the indigo is destroyed. We must first determine how many measures of a liquor, the goodness of which has been ascertained by experiments made upon cloth, are necessary to destroy the colour of one measure of the solution of indigo, and this number will serve to estimate the respective strength of all the liquors which it may be necessary to compare with it. M. Watt employs, in the same manner, a solution of cochineal."

M. Berthollet recommended the following method of procuring the oxy-muriatic acid: "If we have good oxyd of manganese, formed in small crystals, and containing but little extraneous matter, the proportions of the substances to be submitted to distillation are the following: Six ounces of calx of manganese reduced to powder; one pound of common salt, also reduced to powder; twelve ounces of concentrated sulphuric acid, or oil of vitriol; from ten to twelve ounces of water.

"When these materials are prepared, we must carefully mix the oxyd of manganese with the common salt, and introduce the mixture into the distilling vessel placed upon a sand bath: we must then pour upon it the sulphuric acid, previously diluted (and of which the heat occasioned by its mixture with water is dilipated), and immediately apply to the mouth of the matras the Bleaching tube which is to conduct the gas into the intermediate vessel.—It must not be forgotten that in this operation the lutes require particular attention.

"The size of the vessels should be such, that the distilling matras may be about one third empty; and, for the quantity above mentioned, the tub should hold 120 quarts of water; there should also be an empty space of about 10 quarts, in order that when the gas lodges itself in the cavities intended to receive it, the water may have a free space to rise in.

"Before the commencement of the operation, the pneumatic tub must be filled with water. The mixture being made, the gas, which very soon begins to disengage itself, drives out the atmospheric air which is in the apparatus; when it is judged that the atmospheric air has passed into the cavities, it is to be drawn off by means of a bent tube, which is to be introduced successively under each cavity: to drive out the water which has entered into the tube, this last is to be forcibly blown into. The operation is then suffered to go on without fire until it is perceived that the bubbles come over but slowly; then a little fire is to be applied, which is not to be hastily increased at the beginning, but may be gradually augmented, so that at the end of the operation the matter may be brought to a boiling state. It is known to be nearly finished when the tube by which the gas is disengaged, and the intermediate vessel, become hot. When the gas is disengaged only in a small quantity, the fire may be withdrawn; and when the distilling vessel retains but a gentle warmth, it is to be unluted, and warm water is to be poured upon the residue, that it may remain in solution, and thereby be more easily poured out.

"The operation is longer or shorter according to the quantity of materials: with that above mentioned, it should last five or six hours; it is proper not to halter it, that a larger quantity of gas may be drawn off. A single person is able to manage several distillations at the same time; to each of which may be given much larger quantities of materials than those which have been pointed out.

"The intermediate vessel by degrees becomes filled with a liquor which is pure, though weak, muriatic acid; nevertheless, we may perform the operation several times without extracting it; but when it is supposed that there is not sufficient empty space, this acid is to be drawn off by means of a syphon, and, when we have collected a sufficient quantity of it, it may be substituted for the mixture of vitriolic acid and common salt in the operation we have described, if we have no other use to make of it. That there may pass but a small quantity of muriatic acid, not oxygenated, the first tube ought to form a right angle, or even an obtuse one, with the body matras.

"During the operation, the agitator must be from time to time put in motion, to favour the absorption of the gas by the water: when it is finished, the liquor is of a proper strength to use in bleaching; or we may put a less quantity of water in the tub, and then dilute the liquor according to the proportion already mentioned.

"In this state of concentration, although the liquor has a pretty strong odour, it nevertheless is not hurtful, nor even very unpleasant, to those who use it; it is, however, Bleaching, however, proper to conduct it into the troughs where the cloths are placed by means of wooden canals, which are to be connected with the faucet or tube which is at the lower part of the tub."—The following is a description of the apparatus:

ABC D is a reverberatory furnace, having, on a line with B, many small openings in its circumference, to serve as chimneys; within which, upon a sand-bath a, is placed a matras b, the neck of which stands out above the furnace, running thro' the opening D, which is to be closed with clay. The mouth F, of the neck of the matras, is closed by a cork G, through the middle of which passes a tube H, which forms a communication between the inside of the matras b, and the intermediate vessel K, where it also passes through a cork I, which closes one of the three openings of that vessel. The corks G and I ought to be prepared before-hand, and well fitted to each end of the tube of communication H, which is to be so disposed that it may be fitted immediately after the mixture is made in the matras.

The intermediate vessel K is about an eighth part full of water; into it is plunged the tube of safety L, to prevent danger from regurgitation. This tube ought to be so high, that the weight of the water which enters into it, by the pressure of the gas, may be great enough to cause the gas to pass into the pneumatic tub NOP, by the tube of communication M, which is plunged therein, and reaches to the bottom, where it is bent horizontally, so that the gas may be emitted under the first of the three wooden, or (if they can be procured) stone-ware, cavities, or receivers, which are placed in the inside of the tub, one above the other. O is a handle which serves to turn the agitator E, the movement of which facilitates the combination of the gas with the water. P is a spigot and faucet to draw off the liquor.

It is necessary to prepare the cloth by leaving it to soak for 24 hours in water, or, which is better, in some old ley. Afterwards it should be submitted to the action of one or two good leys; because all the colouring part which may be extracted by the leys would else, without any advantage, consume a part of that liquor which it is important to be as sparing of as possible. After this, the cloth is to be carefully washed; then it is to be placed in the troughs, without any part being pressed or confined, in such a manner that it may be thoroughly impregnated with the liquor which is to run thereon. The troughs, as well as the tub, ought to be constructed without iron; for that metal, being rusted by the oxygenated muriatic acid, would produce iron moulds, which could not be taken out but by means of salt of tartar.

The first immersion ought to be longer than the others; it may be continued for three hours, after which the cloth is to be taken out; it is to be again submitted to the action of ley, and then placed in a trough, that fresh liquor may be poured thereon: it is sufficient that this immersion, and the following ones, continue half an hour. When the cloth is taken from the trough, the liquor is to be wrung out, it is to be again submitted to the ley, and afterwards to fresh immersions. The same liquor may serve until its strength is exhausted: when it is much weakened, there may be some fresh liquor added to it. When the cloth appears white, except some black threads and the lints, it bleaching is to be impregnated with black soap, and then strongly rubbed; after which it is to be submitted to the last ley and the last immersion. We cannot determine what number of leys and immersions may be necessary, because it varies according to the nature of the cloth; nevertheless, the limits of this number are between four and eight for linen or hempen cloths.

The manufacturers at Javelle, to whom Mr. Berthollet had communicated this process, soon after published, in different journals, that they had discovered a particular liquor which had the property of bleaching cloth by an immersion of some hours only. The change they had made in the process performed in their preference, consisted in putting some alkali into the water which receives the gas; this enables the liquor to become much more concentrated, so that it may be diluted with several times its own quantity of water before it is used.

"These are the proportions which yielded me (says Berthollet) a liquor similar to the pretended Javelle ley; two ounces and a half of common salt, two ounces of sulphuric acid, six drachms of calx of manganese, and, in the vessel where the gas is to be concentrated, one pound of water, and five ounces of potash, which should be dissolved in the water. The Javelle liquor has a somewhat reddish appearance, occasioned by a small quantity of manganese, which either passes in the distillation, because an intermediate vessel is not used, or exists in the potash; most kinds of which contain it, as I have well convinced myself."

"This liquor may be diluted with from ten to twelve parts of water; and, after this, it bleaches more speedily than the liquor itself; but without speaking of the imperfections of the method which is described in the publications from Javelle, and which can only suffice for cotton, we are not able to bleach near the same quantity of cloth with the oxygenated muriatic acid combined in this manner with an alkali, as might be bleached with the same quantity of that acid mixed with water alone; because there is formed a portion of that neutral salt which is known at present by the name of oxygenated muriate of potash, and in which the oxygen becomes concentrated. Now all the oxygen which enters into the composition of this salt is rendered useless for bleaching; because the oxygenated muriate of potash does not destroy colours."

This method of bleaching was very soon adopted in Britain, and is now almost universal among bleachers. A great many changes have been made in the process; one of the most important of which is substituting lead vessels for wooden ones, which, besides weakening its action exceedingly, were very soon destroyed by the acid. We believe, too, that the bleachers very generally add some alkali to the acid, notwithstanding the strong objections which Mr. Berthollet has made to that manner of bleaching.

This method of bleaching has been found to answer remarkably well; the only objection that has been made to it is, that the cloth is apt to be weakened. And this, no doubt, must be the case, if care be not taken to prevent the acid from being too much concentrated; but we have little doubt that, with a sufficient degree of caution, it will prove as safe as any other whatever; and, in point of expedition, there cannot surely be any comparison drawn between the old mode of bleaching and the new.

It remains for us now to consider, whether the new discoveries in chemistry do not throw some rays of light upon the theory of bleaching; for it is only by perfecting the theory that we can advance with certainty in our practical improvements.

It has been already observed, in the article BLEACHING (Encyc.), that cloth, after being bleached, was a good deal lighter than it had been before that operation: It follows, therefore, that it must have been deprived of something during the bleaching. Cloth bleached by means of the oxy-muriatic acid likewise undergoes a loss of weight; so that, in all probability, both modes act in precisely the same manner.

If raw linen or thread be boiled in a solution of caustic alkali, properly diluted, it gives out something which tinges the ley of a deep brown, and at the same time the alkali loses its causticity. If the linen be boiled in another similar solution, it communicates the same colour, and even a third may be slightly tinged; but after this, alkalis, unless too much concentrated as to injure the texture of the cloth, have no effect on it whatever. If the linen be now plunged into oxy-muriatic acid, properly prepared, and allowed to remain till it begins to become white, and then plunged into an alkaline ley, the alkali loses its causticity, and assumes the same deep colour that the first ley did. Here, then, we have two alkaline solutions; the one saturated with colouring matter before the action of the oxy-muriatic acid on the linen, the other after it. When these solutions are saturated with an acid, a yellow coloured precipitate is obtained, which when dried assumes the appearance of a black powder. Precisely the same substance is obtained from both solutions. This colouring matter is almost insoluble in water. Pure or caustic potash dissolves about double its own weight of it; carbonate of potash not so much.

Hence we see the use of alkalis in bleaching. The colouring matter is not soluble in water, but part of it is soluble in alkali. However, after the alkali has exhausted all its power, the linen is not white; colouring matter, therefore, exists in it, which alkalis cannot act upon. But after being plunged in oxy-muriatic acid, it also becomes soluble in acids. Here, then, is the use of that acid in bleaching—it communicates something to the colouring matter which renders it soluble in alkali. This something, we have already seen, is oxygen. It follows, therefore, that before the greater part of the colouring matter of linen can be extracted by alkalis, it must be combined with oxygen. It is in producing this combination that the use of the exposure to the sun and air consists; and it is because the oxy-muriatic acid produces it almost instantaneously, that the new mode of bleaching is so much more expedient than the old.

If into the alkaline solution of the colouring matter lime-water be poured, there takes place a copious precipitate, which consists of the lime and colouring matter combined. Lime, therefore, has a stronger affinity for the colouring matter than alkali has; and as the compound of lime and the colouring matter is not very soluble in water, lime-water might be used to deprive the alkaline ley of the colouring matter which it has imbibed; after which it might be used again. Care, however, must be taken that no lime-water remains in the ley; otherwise it might precipitate and fix the colouring matter on the linen, after which it would be very difficult to remove it.

From an alkaline ley, saturated with the colouring matter of linen yarn, Mr Kirwan, by means of muriatic acid, precipitated the colouring matter. He found it to possess the following properties: When suffered to dry for some time on a filter, it assumed a dark green colour, and felt somewhat clammy like moist clay. "I took (says he) a small portion of it, and added it to six times its weight of boiling water, but not a particle of it was dissolved. The remainder I dried in a land heat; it then assumed a thinning black colour, became more brittle, but internally remained of a greenish yellow, and weighed one ounce and a half.

By treating eight quarts more of the saturated ley in the same manner, I obtained a further quantity of the greenish deposit; on which I made the following experiments:

1stly, Having digested a portion of it in rectified spirit of wine, it communicated to it a reddish hue, and was in a great measure dissolved; but by the affusion of distilled water the solution became milky, and a white deposit was gradually formed; the black matter dissolved in the same manner.

2ndly, Neither the green nor the black matter was soluble in oil of turpentine or linseed oil by a long continued digestion.

3rdly, The black matter being placed on a red-hot iron, burned with a yellow flame and a black smoke, leaving a coaly residuum.

4thly, The green matter being put into the vitriolic, marine, and nitrous acids, communicated a brownish tinge to the two former, and a greenish to the latter, but did not seem in the least diminished.

Hence it appears, that the matter extracted by alkalis from linen yarn is a peculiar sort of resin, different from pure resins only by its insolubility in essential oils, and in this respect resembling lacca. I now proceed to examine the power of the different alkalis on this substance. Eight grains of it being digested in a solution of crystallized mineral alkali, saturated in the temperature of 60°, instantly communicated to the solution a dark brown colour; two measures (each of which would contain 11 pennyweights of water) did not entirely dissolve this substance. Two measures of the mild vegetable alkali dissolved the whole.

One measure of caustic mineral alkali, whose specific gravity was 1.053, dissolved nearly the whole, leaving only a white residuum.

One measure of caustic vegetable alkali, whose specific gravity was 1.059, dissolved the whole.

One measure of liver of sulphur, whose specific gravity was 1.170, dissolved the whole.

One measure of caustic volatile alkali dissolved also a portion of this matter.

The colouring matter of cotton is much more soluble in alkali than that of linen; hence the greater facility with which cotton is bleached.

From these observations, the great importance of alkalis in bleaching, and the necessity of regulating the strength, Bleaching strength, and ascertaining the purity of the leys made use of, must be apparent. Manufacturers, therefore, lie under very great obligations to Mr Kirwan, who has lately examined the alkaline matters used in bleaching with his usual accuracy and abilities. The result of his experiments was as follows:

| Table of the quantity of mere alkali in 100 Avordupois pounds of the following substances. | |-----------------------------------------------| | One hundred lbs. | Mineral Alkali | | Crytallized soda | 20 lbs. | | Sweet barilla | 24 | | Mcaly's cinnamara kelp | 3,437 | | Ditto desulphurated by fixed air | 4,457 | | Strangford kelp | 1,25 | | One hundred lbs. | Vegetable Alkali | | Danzig pearl ash | 63,13 lbs. | | Clarke's refined ash | 26,875 | | Calshup | 19,376 | | Common raw Irish weed-ash | 1,666 | | Ditto slightly calcined | 4,666 |

When linen is allowed to remain for some time in oxy-muriatic acid, it becomes white. It is evident, then, that when the colouring matter of linen is saturated with oxygen, it becomes colourless; but linen bleached in this manner very soon becomes yellow, especially when exposed to heat. Berthollet, to whose ingenious experiments and observations we are indebted for the greater part of the above remarks, has given the following explanation of the cause of this change: He distilled the colouring matter of linen, and obtained a thick oil, a little ammonia, and $\frac{1}{2}$ of carbon remained behind. The oil contained carbon; and he supposed that carbonic acid gas, and carbonated hydrogen gas, were disengaged. He concluded in consequence, that one-third of this colouring matter was carbon. The other ingredient in the oil was hydrogen; for Lavoisier has proved that oil is composed of oxygen and hydrogen. The colouring matter of linen, then, is composed principally of carbon and hydrogen.

Oxygen combines with hydrogen at a lower temperature than it does with carbon; for if a considerable quantity of oxy-muriatic acid be mixed with a solution of sugar (a substance which consists chiefly of carbon and hydrogen), and the liquor be evaporated, there remains behind little else than carbon, the hydrogen having combined with oxygen and formed water, which had passed off in the form of vapour. Now, whenever a quantity of hydrogen is separated from a body principally composed of hydrogen and carbon, that body assumes a brown or yellow colour, because the carbon becomes predominant; and this colour becomes the deeper the greater the proportion of the carbon compared to that of the hydrogen; and at last, when nothing but carbon remains, it becomes quite black.

It is probable, then, that when the oxy-muriatic acid renders linen white, a quantity of oxygen has combined with the colouring particles, but that this oxygen gradually enters into a combination with the hydrogen, and forms water which passes off; that then the carbon becomes predominant, and the linen in consequence assumes a yellow colour.

The same method does not succeed in bleaching wool and silk which answers for linen and cotton. One would be disposed to think that these substances are bleached rather by losing oxygen than by absorbing it.

Wool, for instance, is rendered white very quickly when exposed to the fumes of sulphurous acid, which we know has a strong affinity for oxygen, and soon fatigues itself with it. But what passes during the whitening of animal matters has never yet been properly enquired into, though it would not only greatly elucidate bleaching, but dyeing likewise, and throw much light upon some of the obscurest parts of chemistry. A great improvement, however, has lately been made by M. Baume in the manner of bleaching silk. Of this improvement we shall proceed to give an account.

Before the silk is wound off the cocons in which the silk worms are inclosed, it is necessary to kill the insects, otherwise they would in all probability eat thro' it and destroy it. This is commonly done by exposing the cocons, properly wrapped up, for two hours to the heat of about 158 degrees of Fahrenheit in an oven; after which they are kept for a certain time in a mass which last to preserve their heat, and effectually destroy such insects as might have escaped the power of the oven. The effect of this process is, that the silk is hardened, and is more difficult to wind off than before. Hence the product of silk is lost by one ninth part in quantity, and inferior in quality to what might have been obtained by winding off without this previous baking.

Mr Baume, not only from these views, but likewise because the silk which has not been baked proves susceptible of a greater lustre, was induced to destroy the crysalis by spirit of wine. For this purpose he deposits them in a wooden box in a stratum six inches deep; upon each square foot half a pint of spirit of wine is to be sprinkled with a small watering-pot made for that purpose. The liquid is to be equally distributed, but it is not necessary that all the cocons should be wetted. They are then to be mixed by hand. In the next place another stratum is to be formed over the first, nearly of the same depth, which is to be sprinkled and treated as before. By this method of proceeding, the box becomes filled, and must then be covered, and left for 24 hours, during which time they become spontaneously heated to about 100 degrees, and the vapour of the spirit of wine exerts itself with wonderful activity. After this treatment they must be spread out to dry, which happens in a short time, and is absolutely necessary previous to winding off.

The spirit of wine to be used in this operation ought to be of the specific gravity .847, at the temperature of 55 degrees. It is of the greatest importance to use that spirit only which has been kept in vessels of glass, of tinned copper, or of pure tin. Leaden vessels are absolutely to be rejected; wooden vessels tinge the spirit, which gives the silk a degree of colour of considerable permanency, and very inimical to the bleaching process.

The silk is wound off upon a reel, while the cocons are kept immersed in water almost boiling. Upon this part of the process M. Baume remarks, 1st, That the dead cocons must be separated. These are known by the brown or black spots on their surface. 2nd, That well water, which on account of its clearness is almost universally used in the silk manufactories, mostly contains nitric acid, and is extremely prejudicial to the bleaching process. The presence of nitrous acid gives a yellow colour, which resists bleaching and even scouring; he therefore... Bleaching therefore recommends river-water. In some countries a small quantity of alum is used. Neither this nor any other saline substance is of the least advantage to the colour, beauty, or quality of the silk.

At the four places of contact of the silk upon the reel, all the threads stick together. It is absolutely necessary that this should be remedied. The method consists in soaking the silk in a sufficient quantity of warm water, at about 90 degrees, for about two hours; after which the threads are to be separated by opening the hanks upon a pin, and lightly rubbing the parts which cohere. When the silk is dry, it is to be loosely folded in its original form, and is ready for bleaching.

The silk while wet is soft, and part of its gummy matter is in such a state, that its threads would readily adhere, if wrung while warm for the purpose of clearing it of the water. After such improper treatment there would be no other remedy than to soak it again in warm water.

The apparatus for bleaching the silk consists of a stone-ware vessel, nearly of a conical form, capable of holding about 12 gallons, having a large opening at the one end, and a smaller of about an inch diameter at the other end. Common pottery cannot be used in this operation, because it is soon rendered unserviceable by the action of the muriatic acid, and the stone-ware itself is not very durable. This vessel must be carefully examined, to ascertain that it does not leak in the slightest degree; after which the inside is to be rubbed with a pumice-stone, to clear it of imperfections which might break the threads. A cover of the same material is to be fitted on by grinding; and the smaller aperture, which in the use is the lowest, is to be closed with a good cork, in the middle of which is thrust a small glass tube about a quarter of an inch in diameter; this is likewise stopped with a cork, excepting at the time when it is required to draw off the liquid contents of the jar. A small perforated false bottom is placed within the vessel, to prevent this tube from being obstructed.

Six pounds of yellow raw silk are to be disposed in the earthen pot; upon this is to be poured a mixture, previously made, of 48 pounds of spirit of wine of the specific gravity .867, with 12 ounces of very pure marine acid, absolutely exempt from all presence of nitrous acid, and of the specific gravity 1.114. The pot is then to be covered, and the whole left in digestion till the following day, or until the liquor, which at first assumes a fine green colour, shall begin to assume that of a dusky brown.

The acidulated spirit is then to be drawn off, clean spirit of wine poured upon the silk, and drawn off repeatedly until it passes colourless. The silk is then suffered to drain without stirring it. In this state it is ready for a second infusion.

Forty-eight pounds of spirit of wine, acidulated with 12 ounces of marine acid, is now to be poured on the silk, and the whole suffered to remain for 24 hours or longer, until the silk becomes perfectly white. The time required for this second infusion is commonly longer than for the first; it sometimes amounts to two, three, or even six days, according to circumstances, particularly the temperature and the nature of the silk. Silk which has been in the oven is in general more difficult to bleach.

When the silk has thus obtained its utmost degree of whiteness, the acidulated spirit is to be drawn off into a separate vessel. This fluid is but slightly coloured, and may be used again in the first infusion of other yellow silk, with the addition of six ounces more of marine acid. The receiving vessel is to be removed, and another clean vessel substituted in its place. The silk is then sprinkled with clean spirit, and occasionally pressed down with the hand. As soon as the spirit of wine comes off absolutely colourless, a third infusion is to be made by pouring upon the silk 48 pounds of the pure spirit without acid, which is to remain till the following day; it is then to be drawn off, and reserved for washing other silk after the first infusion.

After the silk has been left to drain, and affords no more spirit, it still retains its own weight of that fluid. This is recovered by sprinkling the silk with a small quantity of very clear river-water at a time. While the water applies itself and subsides along the silk, it drives the spirit of wine before it, so that the first portions which flow from the tube are scarcely diminished in strength. The addition of water is to be continued until nothing but mere water comes off below.

In this situation the silk is found to be well bleached, but still retains a portion of marine acid sufficient to render it harsh to the touch, and after a time brittle. It must be washed off with water. The best method is to put the silk loosely into a coarse woollen bag, which is to be secured loosely in another cloth like a small bed or pillow, then placed in a basket, and left in a running stream for five or six hours; but where the convenience of a stream is wanting, the earthen pot containing the silk is to be covered with a cloth, and water pumped through it for five or six hours, or until that which issues from the lower aperture gives no red colour to the tincture of turpentine. At this period the lower opening is to be closed, and the vessel filled with water, which must be changed once or twice in 24 hours.

Though the mineral acids are the most powerful and destructive of all saline substances, yet they may be applied to silk when diluted with spirit of wine in very considerable doses. In trials made to ascertain the maximum, two ounces of marine acid were added to one pound of spirit of wine, without altering the silk. Two drams of marine acid cause a very perceptible alteration in one pound of silk.

Spirit of wine which has been mixed with nitrous acid cannot be used in bleaching, even though afterwards rectified upon an alkali, because it still retains a portion of nitrous gas. Pure spirit of wine without acid extracts a fine yellow colour from silk, which does not separate for years, even though exposed to the sun's light. Yellow silk, exposed to the sun, loses its colour in a short time. The acidulated spirit which has been used in the infusion of silk, is changed by exposure to the sun, but not in such a manner as to be rendered fit for use a second time. In order to obtain a beautiful white colour, it is essential that the silk should be immersed in a large quantity of the fluid, especially at the first infusion. Without this management it would become necessary to make three infusions in the acidulated spirit. When the first infusion is well managed, the silk will have lost all its yellow colour, and become considerably white, at the same time that the liquor will have begun to change colour a little. As long as it continues of a fine green, it is certain that it has not exhausted exhausted its whole action upon the silk. The duration of this first infusion may be longer or shorter, without inconvenience, according to the temperature. When the temperature is at 77° Fahrenheit, the first infusion is often made in 10 or 12 hours. In small experiments the heat of the atmosphere may be supplied by the water bath; in which case, all the infusions are easily made in the course of a day.

When the first infusion is finished, and the liquor drawn off, the silk appears greenish; the subsequent washings in spirit of wine clear it of the liquor it retained. This sprinkling should be made with the watering-pot, otherwise the quantity poured will be greater, and the management more wasteful.

Pieces of gauze and entire garments of silk have been successfully bleached in this way.

The finest natural white silks are rendered infinitely whiter by this process. Spirit of wine alone has the property of depriving yellow silk of its colour, which it brings to the state of the naturally white silk. In this state the silk is disposed to acquire a greater degree of brightness by a single infusion in the acidulated spirit. This process has its advantages over the other, to which it is also inferior in certain respects; concerning neither of which the author has entered into any detail.

The colouring matter was found to be a resin perfectly animalized, affording by distillation the same products as other animal matters, and the concrete volatile alkali.

Silk whitened by scouring may be dried freely in the air without affecting its lustre. This is not the case with the silk bleached in the gum; if it be left at liberty to dry in the air, it resembles white flax without any lustre. The beauty of this silk consists in its shining brilliancy; to secure which it must be dried in a state of tension. Mr. Baume has contrived a simple machine for this purpose. It consists of a strong square frame of wood standing upright upon feet; the upper horizontal bar is six feet long, and has six iron pins driven through it at equal distances, so as to project on each side for the purpose of receiving twelve bobbins. The lower horizontal bar is moveable up and down in a mortice, by means of a screw at each end; it is furnished with six holes, adapted to receive as many pins to correspond with those above. The skeins of silk are to be dressed and arranged upon wooden pins, as they are taken out of the hack from washing. As soon as there are twelve together, they are to be wrung with a staff; after which the skeins are to be hung one by one upon as many bobbins put upon the upper pins of the square frame. Another bobbin with tails is to be inserted in the lower loop of the skein, and fastened to the corresponding pin of the lower bar, by means of a strap and hook, which need not be described to such as are slightly acquainted with mechanical objects. When the machine is thus supplied with skeins on both sides, the lower bar of the frame is to be pressed down by the screws until the silk is moderately stretched. When it is dry, the screws are to be equally slackened, the skeins taken off, and folded with a slight twist, that they may not become entangled.

To complete the description of this process, it only remains to show how to recover the alcohol, and ensure the purity of the acids made use of.

The alcohol which has been used in bleaching silk is acid, and loaded with colouring matter. In this state bleaching it cannot be again used. There are two methods of distilling it, which have their respective advantages and inconveniences.

By the first the acid is lost; which is saturated with the alcohol potash, in order that the distillation may be afterwards performed in a copper alembic. A solution of potash is to be poured into the acid spirit, and stirred about to promote the saturation. Carbonic acid is disengaged with strong effervescence from the alkali, and the point of saturation is known by the usual test, that the fluid does not redden the tincture of turpentine. The distillation is then to be made in the copper alembic, and the alcohol referred in proper vessels.

In the second process for distilling without alkali, the acid spirit is distributed into a great number of glass retorts, placed in the sand-bath, on the gallery of a furnace. The first product is scarcely acid; but what follows is more and more so, and must be kept in vessels of glass or stone-ware, which become embarrassing on account of their number. The fluid which remains in the retorts has the colour of beer slightly turbid, and contains the greatest part of the marine acid. It must be poured into one or more retorts, and concentrated by heat gradually applied. The first liquor which comes over is slightly red, turbid, and scarcely acid. This is to be thrown away, and the receivers changed. The succeeding product is the colourless marine acid, of an aromatic smell resembling the buds of poplar. The resin of the silk remains in the retort decomposed by the acid. The marine acid thus obtained is weaker than it originally was; which is in fact of little consequence, as it is pure, and may be safely used, either by increasing the dose proportional to its diminished strength, or by concentrating it if required in the usual way. If this distillation be made in a silver alembic, instead of retorts of glass, and a capital and worm of pure tin be annexed, the alcohol will be obtained so slightly acid as scarcely to redden the tincture of turpentine; but it is sufficiently acid to receive injury if preserved in a copper vessel.

As to the acid, Mr. Baume observes, that the marine acid of commerce is unfit for the purpose. It was formerly prepared with the marine salt of the saltpetre manufacturers; and even when it is made with good salt, the decomposition is effected with common vitriolic acid which contains nitrous acid. Marine acid mixed with a small quantity of nitrous acid does not prevent the silk from being beautifully whitened; it even accelerates the process considerably, and in the most satisfactory manner. But the alcohol, every time it is used and rectified, becomes charged with the acid and gas of nitre, which assume the characters of the nitrous analysing liquor. In this state neither distillations nor repeated rectifications from alkali are sufficient to separate the nitrous matter from the alcohol. Then it is that the success of the operator vanishes, with a degree of rapidity equal to the advances which encouraged his hopes at the commencement.

To purify common sulphuric acid, 100 pounds of it are to be mixed in a large basin of copper with the same quantity of river-water, and stirred with a wooden spatula. The mixture instantly becomes heated to the boiling-water point, and a great quantity of red vapour is disengaged, which has the smell of aqua-regia, and arifie Bleaching arises from the nitric and muriatic acids. When this mixture is made, it is proper to immerse the bason to a suitable depth in a large vessel of water, to hasten the cooling. As soon as it is sufficiently cooled, it is to be drawn off into bottles, and left to become clear during several days. It is in the next place to be decanted, and conveyed into retorts by a typhon funnel, and the rectification proceeded upon until it becomes perfectly white. Towards the end of the operation a small quantity of sulphur sublimes in the neck of the retort. Instead of receivers, a small glass cup is placed beneath the aperture of each retort, in order to facilitate the diffusion of the nitric and muriatic acids. When the acid in the retorts is sufficiently cooled, it is poured a second time into the copper bason, and mixed with 100 pounds of river-water, as at first, and again concentrated in the retorts till it becomes perfectly clear. The muriatic acid is to be distinguished from common salt by the application of this acid in the usual manner.

The oxy-muriatic acid is also used very generally for bleaching paper, or rather the stuff out of which paper is made. It has been alleged, and we believe with some truth, that since this mode of whitening paper was introduced into this country, the strength of paper is much inferior to what it was formerly. If this be really the case, perhaps it is owing to the use of too concentrated an acid.

We shall finish this article with Mr Chaptal's account of this process, who was the first person that introduced it. "Blotting paper (says he), by being put into oxygenated muriatic acid, is bleached without suffering any injury; and rags of coarse bad cloth, such as are used in the paper manufactories to make this kind of paper, may be bleached by this acid, and will then furnish paper of a very superior quality. I bleached by it an hundred weight of paste, intended to be made into blotting paper, and the increase of value in the product was computed at 25 per cent. whereas the expense of the operation, when calculated in the strictest manner, amounted only to 7 per cent."

The property possessed by this acid, of bleaching paper without injuring its texture, renders it very valuable for restoring old books and smoked prints. The latter, when discoloured to such a degree that the subject of them could hardly be distinguished, were re-established and revived, in so astonishing a manner that they appeared to be new; and old books, foiled by that yellow tinge which time always produces, may be so completely renewed, that one might suppose them to be just come out of the press. The simple immersion of a print in oxygenated muriatic acid (leaving it therein a longer or a shorter time, according to the strength of the liquor) is all that is required for bleaching it; but when a book is to be bleached, some farther precautions are to be used. As it is necessary that the acid should wet every one of the leaves, the book must be completely spread open, and then, by letting the boards of the binding rest upon the sides of the vessel, the paper only will be immersed in the liquor. If any of the leaves stick together, they must be carefully separated, that all of them may be equally impregnated. The liquor takes a yellow tinge, the paper grows white; and after two or three hours the book may be taken out of the liquor, and soaked in clean water, which should be changed from time to time, in order to wash out the acid with which the book is impregnated, and also to deprive it of the disagreeable smell it has contracted.

"The above method, which is the first I made use of, has generally succeeded pretty well; too often, however, the leaves of my books have had a motley appearance; and sometimes several pages were not at all bleached; I was therefore obliged to have recourse to the following more certain process. I began by unfewing the books, and reducing them into sheets; these sheets I placed in divisions made in a leaden vessel, by means of thin slips of wood, so that the leaves when laid flat were separated from each other by very small intervals. I then put the acid into the vessel, pouring it against the side, that the leaves might not be disturbed; and when the operation was finished, I drew off the acid, by means of a cock fixed in the bottom of the vessel. I then filled the vessel with clean water, which washed the leaves, and took off the smell of the oxygenated acid. They may then be dried, smoothed, and sewn bound. In this manner I have restored many valuable books, which had become worthless from the bad state they were in.

"When I had to bleach prints so torn to pieces that they consisted only of fragments fitted together, and pasted upon paper, I was afraid I might lose some of these fragments in the liquor, because they separate from the paper by the softening of the paste; in that case therefore I took the precaution of enclosing the print in a large cylindrical bottle, which I turned upside-down, fixing its mouth to that of a vessel in which I had put a mixture proper for detaching oxygenated muriatic gas. This gas fills the inside of the bottle, and acting upon the print, takes off the stains, ink-spots, &c., while the fragments remain pasted to the paper, and consequently keep their respective places."