Bleaching and calico-printing : a practical manual

Front matter

FRANKLIN INSTITUTE LIBRARY

PHILADELPHIA, PA.

REFERENCE at ' ■ v

Preface

The subject-matter of this manual consists mainly of the course of lectures (in a form sufficiently condensed to meet the require¬ ments of the publishers) given annually to classes of Students studying the Technology of Bleaching and Calico-Printing under the Author’s direction. Although the subject is necessarily treated technically, yet the aim throughout has been to make it as clear, concise, and simple as is consistent with a thorough scientific explanation of the principles involved. The book is not intended as a mere source of “ cram ” for an examination, but as a reliable and practical guide to the young bleacher and calico-printer. In this the Author has received much valuable assistance from Mr. William Turnbull, who is well known as a practical calico-printer.

The patterns introduced are not highly-finished, elaborate designs, intended to advertise the firms supplying them, but are of the most simple kind possible, and intended to illustrate the various stages of bleaching and calico-printing. The recipes given are not a collection culled from other books, but are those by which the accompanying patterns were actually produced, or which are known to be reliable and practicable.

Chemical equations are freely used for the purpose of illus¬ trating methods of preparing the various mordants, dyestuffs, and other substances necessary for the production of the colours, and the Student is strongly advised to make himself familiar at the outset with the general principles of chemistry and physics.

G. D.

Stacksteads, December , 1895.

in 26

Chapter I. — Bleaching. Page

Cotton, . 1

Composition, .... 2

Action of Chemicals, . . 2

Impurities, .... 4

Singeing, .... 4

Grey Wash, .... 6

Testing the Lime, ... 7

Page

Slaking the Lime, ... 8

Bowking, . . . .10

Grey Souring, . . .10

Ley Boil, . . . .11

Theory of Bleaching, . . 13

Testing Bleaching Powder, . 14

New Bleaching Processes, . . 16

Mather-Thompson Process, . 17

Chapter II. — Printing. Hand -block Printing

. 19 1 Perrotine-Machine Printing,

. 21

Flat Press Work, .

. 21 | Cylinder-Machine Printing, .

. 21

Chapter III. — Mordants

Chemical Elements,

. 27

Mordants, . . . .

. 29

Compounds, .

. 28

. 29

Classification,

. 28

Acetate of Alumina,

. 30

Ferrous Acetate, .

. 31

Chapter IV. — Styles of Calico-Printing. The Dyed or Madder Style, . 32

Dunging or Fixing Process, . 39

The Dyeing, .... 39

Resist Padded Style, ... 41

Discharge Style, .... 42

Extract Style, .... 43

Pigments, ..... 55

Chromed or Raised Colours, . . 56

Insoluble Diazo Colours, . . 60

Chapter V.— -Thickeners

Thickening Materials, .

. 69

Albuminoid Thickeners,

. 71

Amylum or Starch,

. 69

Preparation of Thickeners, .

. 73

Dextrin,

. 71

Straining, ....

Gums, . 71 viii

CONTENTS.

Chapter VI.— Natural Organic Colouring Matters. Logwood

Page

. 75

Old Fustic,

Red Woods, ....

Safflower, &c.,

Yellow Dyestuffs, .

. 76

Indigo, . . . . .

Madder, . . . .

Quercitron Bark, .

. 76

Chapter

VII. — Tannin Matters.

Gall Nuts, ....

. 82

Myrobolans, . . . .

Tannic Acid,

. 82

Valonia, &c.,

Sumach, ....

. 82

Chapter VIII. — Oils — Soaps — Solvents. Oils

. 84

Solvents,

. 84

Acetic Acid, .

Castor Oil,

. 84

Glycerine, .

Preparation of Alizarine Oil,

. 84

Acetin, . . . .

Soaps, .....

Assay and Analysis,

. 85

Ethyl Tartrate,

. 86

Page

Chapter IX. — Organic Acids — Salts. Organic Acids

. 90

Potassium Antimony Tartrate,

Citric Acid, .

. 90

Lead Salts, .

Oxalate,

Tartaric Acid,

. 91

Oxalic Acid, .

. 91

Acetate of Lead,

Salts, ....

. 91

Lead Nitrate,

Stannous Chloride,

. 91

Stannic Chloride, .

. 92

, , Oxide,

. 92

Carbonate of Lime,

Stannate of Soda, .

. 92

Acetate of Lime, .

Stannic Oxalate, Stannous Acetate, .

. 92

. 94

Alumina Salts,

Chapter X. — Mineral Colours or Pigments. White Pigments

Yellow Pigments, .

Red Pigments,

96 Green Pigments,

96 Blue Pigments,

Chapter XI.— Coal-tar Colours. Distillation of Coal Tar, . . 98

Benzene, .... 98

Toluene, .... 99

Xylenes, .... 99

Naphthalene, . . . .100

Anthracene, . . . .100

Aniline and Toluidine, . . 101

Naphthylamine, . . . 102

Aniline Salts, . . . 102

Naphthalene Colours, . . 106

Benzopurpurine, . . .107

Anthracene Colouring Matters, 108 Manufacture of Alizarine, . 108

CONTENTS.

IX

Chapter XII. — Eyeing. Page | Page

Indigo Dyeing, . . . . 114 | Direct Colours, . . . .121

Chapter XIII.- —Water — Theory of Colours

. 125

Theory of Colours,

. 128

Quantity,

. 125

Weights and Measures,

. 132

Clark’s Soap Test, .

. 126

Tables, ....

. 134

Softening Hard Waters,

. 127

Index, .

. 139

Patterns.

LIST OF ILLUSTRATIONS.

PLATES

Many-colour printing machine,

Shearing machine or Canroy, .

Padding mangle, .

FIG.

1. Ripe cotton fibre,

2. Unripe cotton fibre,

3. Sections of cotton fibre, .

4. Double plate-singeing machine,

5. Revolving plate-singeing machine,

6. Apparatus for testing lime,

7. Low-pressure kier, .

8. High-pressure kier, .

9. Washing, liming, chemicking, and

10. Washing machine for bleach work,

11. Working parts of cylinder machine,

12. Preparing machine, .

13. Continuous steam ageing machine,

14. Continuous system of first dunging,

15. Dye-beck,

16. Lancashire jigger,

17. Tapioca starch,

18. Wheat ,,

20. Potato ,,

21. Maize ,,

23. Double-cased copper colour pans

PAGE

8 9

AND

CALICO-PRINTING.

Chapter I.— Bleaching. Cotton

Cotton consists of the fibres which are attached to the seeds of the cotton plants, a branch of the order of mallow worts or Malvaceae; to this branch the generic name of Gossypium is given. There are several species of cotton plants known, the principal being Gos¬ sypium barbadense, which yields the Sea Island and other American varieties of cotton ; Gossypium Peruvianum , which yields the South American varieties of cotton ; Gossypium herbaceum, which yields the short fibre varieties of Indian, China, and Asiatic cotton ; and Gossypium arboreum , which yields the long-fibred Egyptian and Asiatic cottons.

The cotton fibre, when seen under the microscope (see Eig. 1), is essentially a tube of a more or less spiral shape, and may be likened to

Fig. 1. — Ripe cotton fibre.

a ribbon with thick edges twisted on itself. The length of the cotton fibre varies considerably; in Sea Island cotton it reaches nearly 2 inches, but in Indian cottons it is often met with under an inch in length. The diameter is small, and ranges from 0*00046 to 0*001 of an inch.

The cotton fibre will support a weight of from 84 to 164 grains, the fibres having the largest diameters being usually the strongest.

As stated above, and shown in Eig. 1, the cotton fibre when viewed under the microscope appears to resemble a twisted ribbon, the edges of which are thicker than the middle portions. This twisted appear¬ ance is found to be more highly developed in ripe fibres than in unripe fibres. In the latter case the fibres are full of sap, which on drying out causes the fibres to collapse and assume a flat shape, which

BLEACHING AND CALICO-PRINTING.

is seen in Fig. 2. In the centre of the fibre is a tube which is much more visible in ripe than in unripe fibres. It is worth noting that unripe fibres do not attract dye so well as the ripe fibres.

Composition. — Cotton in its natural state is principally composed of cellulose, and is accompanied by a small amount of natural im¬ purities which average about 4 or 5 per cent.

The cellulose of cotton is a white substance belonging to the class of carbo-hydrates — that is, a class of organic bodies consisting of three elements, viz., carbon, hydrogen, and oxygen, the two latter being present in the proportion in which they form water, namely, 1 part of hydrogen to 8 parts of oxygen by weight — so that we may look upon cellulose as a sort of chemical combination of carbon and water : hence the term “carbo-hydrate.”

Action Of Chemicals. — The empirical formula of cellulose is C6H10O6. It is insoluble in ordinary solvents, water, alcohol, ether, &c. ; but it dissolves in strong sulphuric acid, also in an ammoniacal solution of cupric hydrate. Cold dilute mineral acids have no action, but if allowed to dry upon the cotton they gradually become con¬ centrated, and then tender the fibre. The same corrosive action

Fig. 2. — Unripe cotton fibre.

takes place if cotton impregnated with such acids is heated. The process of “extracting” or “carbonising” woollen rags containing cotton ( i.e ., destroying and removing the cotton) by means of sulphuric or hydrochloric acids is founded on this fact. By a brief immersion in a mixture of 1 part strong sulphuric acid and 1 part water, cellulose in the form of paper is converted into vegetable parchment. By the action of a mixture of strong nitric acid and sulphuric acid, cellulose is converted into nitro-cellulose ; a compound in which nitric peroxide has replaced some of the hydrogen of the cellulose. Gun-cotton is one form of nitro-cellulose, and soluble pyroxyline, a substance used in the preparation of collodion, is another. Acetic, tartaric, and citric acids have little or no action on cotton ; a cold solution of oxalic acid has no action, but if cotton be heated therewith to 100° C., it is more or less “ tendered ” ; hence care must be taken when using steam colours containing oxalic acid for printing calico. Cotton is not so readily affected by alkalies as it is by acids, and in this respect is just the reverse of wool. Cotton may even be boiled for several hours with weak caustic alkalies, if care be taken that it is immersed below the surface of the solution ; but under some circumstances the combined action of steam, air, and alkalies, especially lime, will tender cotton ; and such conditions must be guarded against during certain of the operations of bleaching. The action of strong solutions of caustic

COTTON FIBRES.

soda or potash is very remarkable. If a piece of calico be steeped for a few minutes in a solution of caustic soda of about 50° Tw. (sp. gr. 1 *25), it assumes quite a gelatinous and translucent appearance ; when taken out and washed free from alkali, it is found to have shrunk con¬ siderably and become much closer in texture. If a single fibre thus treated be examined under the microscope it is seen to have lost all its original characteristic appearance ; it is no longer flat and spirally twisted, but seems now to be thick, straight, and transparent. A transverse section shows it to be cylindrical, while the cell walls have considerably thickened, and the central opening is diminished to a mere point (Fig. 3).

c

Fig. 3. — Sections of Ripe, Unripe, and Mercerised cotton fibre.

Cotton thus treated is said to be “mercerised;” mercerised cotton has a greater attraction for colouring matters than ordinary cotton. Dilute solutions of hypochlorites — bleaching liquors — do not act injuriously when cold ; but stronger solutions, especially in contact with air, convert cotton into what has been called by Witz, oxycellulose.

Many salts, such as chloride of tin, sulphate of alumina, nitrate of lead, &c., will tender the cotton fibre, especially if it be heated with them, as is sometimes done in the production of steam colours in calico-printing.

Cotton has a strong affinity for tannin ; advantage is taken of this fact for the purpose of mordanting cotton with either tannin or tannin materials, like sumach, divi-divi, chestnut, &c., for dyeing with the basic colours.

Cotton has but little affinity for colouring matters. With a few of the natural dyestuffs, turmeric, safflower, and annatto, it will unite directly. Of the coal-tar colours, cotton has no affinity for the acid and azo colours in general. Of the basic colours, saffranine and methyl violet are the only ones which possess any sort of affinity for cotton, and even these will only produce pale shades. For the so-called Congo or direct cotton colours, of which Congo-red was the first commercial representative, cotton has a very powerful affinity, and with these dyestuffs produces some very deep and fast shades. What particular feature in the composition of these colours gives them this property of direct combination with cotton, is not yet satisfactorily explained.

BLEACHING AND CALICO-PRINTING.

BLEACHING.

Impurities. — By natural impurities we understand everything present in the fibre except cellulose — that is to say, cotton wax, a small quantity of margaric acid, pectic acid, albuminous matter, and colouring substances. The artificial impurities — accidental or inten¬ tional — may amount in woven goods to 20 or 30 per cent. They consist of grease, starch, and other ingredients used in the sizing, besides oil from the machinery. The removal of these impurities so that nothing but pure white vegetable fibre may remain is the result

SINGEING.

sought in the highest grade of bleaching — technically known as the “ madder bleach ” — because it was first applied to goods to be printed with madder colours. It is also used for cloths which are to receive light and pure colours. For goods that are to be dyed black or other dark shades, a less thorough treatment will be sufficient. For goods that are to remain white, a special bleach called a “ market bleach ” is given.

Singeing. — Previous to the bleaching operations proper, piece goods are usually stamped with gas tar, or some other substance capable of resisting the bleaching process, in order that they may be afterwards identified. After being marked, the pieces are stitched together, end to end ; they are then passed through the singeing machine (shown in Figs. 4 and 5) in order that the fine loose down

Fig. 5. — Revolving plate-singeing machine; the advantage of the revolving plate is that a red-hot surface is always presented to the cloth.

’BLEACHING AND CALICO-PRINTING.

may be burned off the surface of the cloth, as this down interferes with the production of a fine impression in the printing process. The singeing is performed by rapidly passing the cloth in the open width over red-hot copper plates or between rows of gas flames ; formerly all goods were singed by the hot plate method, but lately gas singers have come more into use, especially for the lighter and thinner cloths. The singeing plate must be kept at a very strong red heat, and in order to do this a very good fire must be kept up immediately under the plate ; while the cloth is drawn rapidly over one or more of these hot plates by means of rollers, a frame with iron bars depresses the cloth tightly upon the hot plate, and one of the greatest difficulties with the plate singeing machine is to get the cloth to press evenly upon all parts of the plate, so that one part of the cloth may not be burned more than another, otherwise the cloth will show streaks after it has been dyed a plain shade ; of course this difficulty is not experienced with the gas singeing machine, which consists essentially of one or more rows of Bunsen burners, through which the cloth is drawn at a rate suitable for the kind of cloth to be singed. After passing over the singeing plate or between the gas burners, the cloth should immediately pass through a water trough, or else through a pair of wet draw bowls, in order that any sparks may be at once extinguished.

Grey Wash. — The pieces are drawn direct from the singeing house guided by means of glazed earthenware rings, “pot eyes,” through the washing machine and plaited down, “in pile,” on a stillage in the bleach house, where they are allowed to lie a few hours to soften. This, the first stage in bleaching proper, is called the “grey wash,” and the pieces previously in the open width have now assumed the rope form, which they will retain throughout the whole of the bleach¬ ing operations.

The Lime Boil. — The second process in bleaching, called the “lime boil,” is a very important one. As already stated, the cotton fibre is made up of cellulose and a small quantity of other substances, such as wax, pectose, and colouring matters ; now it is very probable that the object of the lime boil is to break up the combination of the wax and the cellulose ; for it has been found in practice that nothing is so effective for the removal of the waxy coating from the cotton fibre as boiling with lime. The lime for this purpose should be freshly burned and well slaked ; if the lime is old it will have absorbed carbonic acid from the atmosphere, and this will render it unfit for bleaching pur¬ poses. It will also be found that lime which has been exposed to the atmosphere cannot be slaked like freshly-burned lime. The lime should be well slaked ; for badly-slaked lime will be in a very porous condition, almost like sand ; in this porous condition it very quickly absorbs carbonic acid gas from the atmosphere, and so becomes spoiled for the purpose of bleaching. On the other hand, if the lime be good and well slaked, it will have the consistency of fat, in which

LIME BOIL.

state it is very impervious, so that the carbonic acid gas cannot enter into and combine with it, and in this condition it will keep good for months. I believe this exclusion of the carbonic acid to be a very important matter, and one which (owing to the invisible nature of the gas) is liable to be overlooked by the workmen. In fact, sa deceptive is the appearance, that even an experienced chemist cannot distinguish between good and bad lime by merely looking at it ; he must apply some chemical test.

Testing* the Lime. — As stated, the lime may have absorbed a considerable amount of carbonic acid gas, either before or after being slaked, and this can only be found by treating the sample with nitric acid in a suitable apparatus, so that the amount of evolved carbonic acid may be determined by weighing. The form of apparatus most convenient for this purpose (shown in Fig. 6) consists of two light glass bottles, of about half a pint capacity, with perforated indiarubber stoppers fitted to each. The bottles are connected by a glass and indiarubber tube, the extra hole in each stopper being plugged with a solid glass plug. The amount of lime which should be weighed off is about 10 grammes ; this, after weighing, is slaked with water, and then poured into bottle B. Next, a tube, holding about 100 c.c., is about two-thirds filled with moderately strong nitric acid (1 part acid, 1 part water), and is then carefully lowered into bottle B, taking care not to spill any acid from the tube. The tube must be of such dimensions as to rest in the bottle just in the manner shown in Fig. 6, without being able to fall oVer. Bottle A contains strong sulphuric acid up to about one-third of its capacity. The india-rubber tubes and stoppers are now attached to both bottles, so as to connect them. Both bottles and contents are next carefully weighed together, and after weighing, bottle B is tilted on one side, so as to cause the acid to run out of the test tube on to the lime. This will cause the whole of the lime to dissolve, but no gas will be given off if it is quite pure. However, as the lime is not likely to be quite pure, it is almost certain there will be more or less gas given off, and this gas will pass out of bottle B, through the tube into bottle A, and bubbling through the sulphuric acid, will finally escape through the small hole in the stopper, the plug of which has been previously with¬ drawn. After about ten minutes, when all action has ceased, the bottles must be placed in a dish of hot water ; a small bit of glass tube (about 2 inches long), to which is attached 1 foot of indiarubber tubing, is inserted air-tight into the hole of the stopper of bottle A,

Fig. 6. — Apparatus for testing lime.

BLEACHING AND CALICO-PRINTING.

the other end being placed in the mouth; suction is applied, and at the same moment the stopper of bottle B is removed ; this will cause a current of air to be drawn through the apparatus, which will sweep out the whole of the carbonic acid. This being done, the aspiration tube is removed from bottle A, the two glass plugs are replaced, the bottles removed from the water, allowed to cool, and dried ; the plugs are then again removed for a moment and replaced.

Fig. 7. — Low-pressure kier, which can also be used as an open kier.

The whole apparatus and contents being reweighed, the difference between this and the first weighing- is the amount of carbonic acid in

o o

the sample of lime. Should this amount to 1 gramme, the lime is not good. This is a very practical and accurate way of testing the quality of the lime.

Slaking* the Lime. — Having selected a good sample of lime, it is next well slaked as follows : — Into a rather deep and water-tight pit,

LIME BOIL.

run in water through a trough or pipe, at the same time throwing in lime at a speed sufficient to keep the water in a vigorous state of ebullition, taking care to keep the pieces of lime well raked up from the bottom of the pit, in order that it may all be thoroughly slaked. For every ton of cloth to be bleached, about 1 cwt. of slaked lime

Fig. 8. — High-pressure kier worked by injector.

will be required, and this must be mixed with sufficient water to make it into a thin milky liquid, which must be run through a sieve into the liming box, as great care must be taken that no large particles of solid matter may come in contact with the cloth ; other¬ wise much damage may be done to the cloth in the liming machine.

IO

BLEACHING AND CALICO-PRINTING.

Cases have occurred where, in consequence of a piece of crock or a nail having got into the squeezing bowls of the liming machine, many hundreds of yards of the cloth were damaged before it was detected.

Bowking*. — -Having been run through the milk of lime in the liming machine, the cloth is passed on into the kier, where it will be subjected to a “ bowking” or boiling. In filling the kier with cloth, care must be taken that it is packed in very systematically ; this is done by boys who go into the kiers when they are being filled, and with their hands, and wooden clogs on their feet carefully tread down the cloth in such a way that it will not get tangled during the boiling process. The quantity of lime water must be regulated to the amount of cloth in the kier, neither so much that the goods will be floating about, nor so little that they may get dry, and so become lime-burned. The time required to complete the lime boil will depend upon the kind of kier and the pressure ; in closed kiers worked at a pressure of 50 lbs. the boiling may be completed in 6 or 8 hours ; in low-pressure kiers, worked at about 10 lbs., twelve to sixteen hours may be required with open kiers, twenty-four or even thirty hours may be given (see Figs. 7 and 8). After the goods have been properly boiled, the waste liquor is run off from the bottom of the kier, and the cloth is drawn by the draw bowls of the washing machine from the kier. At the same time much of the lime and dirt is removed from the cloth during its passage through the washing machine ; but as only a portion of the lime can be removed in this way, the cloth is next subjected to a treatment with weak acid called the “ grey sour.”

Grey Souring’. — The acid generally used for this purpose is hydrochloric at from 1° to 2° Tw. ; this acid, usually called “salts,” is preferred, as it is cheap and forms a very soluble salt with the lime, viz., calcium chloride (CaCl2), which can be easily removed from the cloth by a subsequent washing ; this washing should be continued until all the acid and calcium chloride has been removed from the cloth. The “ Twaddell glass ” used for determining the density of liquids is a very useful instrument when we know that we are dealing with pure chemicals, but should the liquids be adulterated, this instrument will neither detect the kind nor the amount of the impurities, and in fact it then becomes a useless or even a very misleading instrument. The best way that I know of for a bleacher to test his salts is as follows : — Take 1 pint of the salts to be tested, add 2 pints of water to it, then weigh off 36|- grms. of the mixture in a small dish or basin ; next weigh a piece of marble (any weight from 50 grms. to 500 grms. ; it may be obtained from any dealer in analysts’ chemicals for about 3d.), carefully drop the weighed piece of marble into the diluted salts in the basin, leaving it there for twelve hours or so ; then take out the marble, wash it carefully (so as not to break any bits off) in water, put it into a

LEY BOIL.

I I warm place for a few minutes to dry, then reweigh it, and if the salts are good the piece of marble will have lost about 5 grms. ; if the marble loses much less than this quantity, say only 4 grms., the salts are only four-fifths the strength they ought to be.

Boiling* with Soda Ash and Resin Paste.— The next process in the madder bleach is boiling with soda ash and resin paste. This process is just a repetition of the lime boil, only using soda ash and resin paste instead of lime. For every ton of cloth, about 1 cwt. of soda ash and 30 lbs. of resin will be required. It is best to boil these substances together in a pan, with about 100 gallons of water, as in this way the resin will be converted into a kind of soap, solublejin

Fig. 9. — Washing, liming, chemicking, and souring machine (ordinary form).

water. This solution of resin is run into the kier along with sufficient water to thoroughly scour the cloth ; this scouring will, in the high-pressure kier working up to 40 lbs., be completed in about six hours ; but in the low-pressure or open kiers, twelve hours’ boiling will be necessary. After a thorough boiling, the spent liquor is run off from the bottom of the kier, the goods are then turned over through the washing machine (see Figs. 9 and 10) and again boiled in a solution of

BLEACHING AND CALICO-PRINTING.

soda ash (30 lbs. per ton of cloth) without the addition of resin. This second ash boil will only require about half the time necessary for the first, and its object is twofold — first, to still further remove impurities from the cloth, and, secondly, to remove any resin paste which may have been left in the cloth by the first ash and resin boil ; as, if any

Fig. 10. — Washing machine for market bleach work, or squeezing machine.

resin were left in the cloth when the goods are drawn out of the kier, it might become permanently fixed there by the subsequent treatment which the cloth has to undergo. After the cloth has received another washing, the only impurities remaining in it will be a small amount of colouring matter, and it is in order to remove this last impurity that the cloth is subjected to the process called “ chemicking.” It will perhaps be as well here to study the theory of the process of bleaching.

THEORY OF BLEACHING.

THEORY OF BLEACHING.

In the first place, then, it would not do to “chemic” the cloth before it has been bowked with lime and with soda. There are certain impurities in the cloth which must be removed before the chemicking can be done satisfactorily, and these impurities cannot be removed by soda alone, for, although it is not very clear why, it is found in prac¬ tice that it is best to begin with a lime boil first. It is thought by some that the lime combines with the waxy coating of the cellulose, so as to form a compound of lime and wax ; but as this lime compound is not soluble in water, it cannot be removed from the cloth ; however, when the lime is removed by souring, it leaves the wax in a state in which it will more readily combine with soda to form a soluble com¬ pound. It is not clearly understood what part the resin plays in the “soda-resin boil,” but it may be that a kind of triple compound of soda-resin-wax, soluble in water, is formed. It is certain that impur¬ ities cannot be removed from the cloth until they have been rendered soluble in water, and it is tolerably certain that during the various bowkings the impurities are rendered soluble, and so removed from the cloth. Previous to the invention of bleaching powder, cloth was always bleached by laying it upon growing grass, in so-called bleachcrofts ; the growing grass has the power of decomposing the carbonic acid always present in the atmosphere ; the grass retains the carbon of the carbonic acid, but throws off the oxygen in a nascent condition, and oxygen in this state has the power of destroying almost every kind of colouring matter. Of course the action of sunlight would hasten the bleaching very considerably; nevertheless, this was a very slow though sure process of bleaching. The modern process of bleach¬ ing is due to the discovery of chlorine gas ; not that chlorine gas can really bleach — that is, decolorise colouring matter ; but chlorine gas can, under certain conditions, decompose water, and so produce nascent oxygen, which, as just mentioned, has the peculiar property of destroy¬ ing colouring matter, or bleaching. Now chlorine is only very loosely combined in the chemic, and is, therefore, very easily set free; all acids, even carbonic, will decompose chemic. Take a piece of cloth which has been dyed with coal-tar colour, say alizarine or magenta ; dip it into a solution of chemic, and then into some sours, the colour is rapidly bleached, and, at the same time, a strong odour of chlorine gas is given off. Now, every bleacher is aware that there are two objections to this method of bleaching. In the first place, the stench and irritating action of the chlorine are so powerful that no one would be able to stand them for long ; and, in the next place, it would be found the cloth would have been more or less tendered by the too rapid action of the chemicals. It is in order to avoid this evolu¬ tion of chlorine, and consequent tendering, that the cloth is allowed