The negative and print, or, The photographer's guide in the gallery and in the field : being a text-book for the operator and amateur, containing brief and concise instructions for the preparation of the different kinds of photographs now in vogue. Such as the ordinary negative for contact printing, the solar negative and the negative for the copying camera, printing on albumen paper, vignette printing, printing positives on opal or plain glass, the ambrotype, ferrotype, melainotype etc., stereographic printing, the dry process, the construction of the glass-house, the camera and the lens, and the treatment of the gold and silver residues

Front matter

FRANKLIN INSTITUTE LIBRARY

PHILADELPHIA

FcL £ and to holders of second class stock, any work belonging to the second class, subject to the following regulations :

Chapter I

Preparation of the Glass Plate for the Reception of the Col¬ lodion Film — Solutions for Cleaning Glass Plates — Chro¬ mic Acid Bath — Albumen in Stock Solution, and Diluted for the Substratum — To Albumenize the cleaned Plates, 13

Chapter II

Preparation of the Collodion, and the Mode of Coating Albumenized Plates with Collodion — To Prepare Pyroxylins by the two Methods — Preparation of Plain Collodion — Preparation of Bromo-Iodized Collodion — Preparation of Iodized Collodion — Vials for Collodion — Pneumatic Holders, . . . . .10

Chapter III

The Nitrate of Silver Bath, its Preparation, Troubles, and Renovation — Neutralizing the Bath with Carbonate of Soda ; with Oxide of Silver — Boiling the Bath — Distilling oft the Ether and Alcohol — Decomposition of the Nitrate of Silver in an Old Bath, . 29

Chapter IV

The Sky-Light, Side-Light, Screens, and Accessories for Po¬ sing the Model — Theoretic Views about Lighting — Practi¬ cal Construction of a Photographic Glass-House, . . 38

Chapter V

Lenses and Cameras — The Use of Stops or Diaphragms — Spherical Aberration ; Chromatic Aberration — their Cor¬ rections — Foreign Lenses — American Lenses — To Make the Actinic Focus and the Luminous Focus Coincident —

To Focus, . 47

306 /

X

CONTENTS.

Chapter VI

The Dark-Room — Development— Non-Actinic Room — Transparent Dish for Holding the Plate during De¬ velopment, . 58

Chapter VII

Developers — Intensifies — Theoretic Views of the Develop¬ ment — Developer — Art of Development — Benefit of the Albumen Substratum — Fixing Solution — Intensification and Toning of the Negative, . 62

Chapter VIII

To Varnish Negatives — To Prepare Negatives for the Solar Camera and for Copying with a Camera — Collodion for the Solar Camera Negative — Method for Collecting the Silver from the Wash Water, etc. — Fixing Solution — Pyrogallic Acid Developer — Collo-Protosulphate of Iron Developer, . . 72

Chapter IX

The Print, how to Make It — Preparation of the Paper — Sen¬ sitizing of Albumen Paper — Fuming of Sensitized Paper — The Printing Operation — To Prepare Terchloride of Gold — Preparation of the Toning Bath — Fixing Solution for Prints — Construction of the Washing-Tank — The Drying-Frame — Mounting of Prints, . . . .82

Chapter X

The Negative by the Dry Process — The Dry Tannin Process — Acid Development — Alkaline Development — Tannin Preservative — Time of Exposure, . 100

Chapter XI

To Prepare Transparent Positives — Positive Printing on Glass by Means of the Camera — To Prepare Transparent Positives by Contact Printing— Varnish for Transparent Positives — To Prepare Dry Plates by Gordon’s Process —

To Prepare Transparent Stereographs, . . . .107

CONTENTS.

XI

The Magic Print, . . . . . . .142

Chapter XVII

What to do with the Silver and Gold Residues — To Reduce Chloride of Silver by the Wet or Galvanic Process to Pure Silver — To Reduce the Chloride or Oxide of Silver by Fire — To Reduce the Gold Residues, .... 144

CHAPTER XVIH.

Landscape Photography — Tents for Landscape Photograyhy, 148

THE

PHOTOGRAPHER’S GUIDE.

Chapter I

PREPARATION OP THE GLASS PLATES FOR THE RECEPTION OF THE COLLODION FILM.

It is almost unnecessary to assert, that the finest and clearest plate glass is the best adapted for photo¬ graphic purposes, the only objection is the expense. If the practical photographer lays in a stock of glass in large plates, it becomes necessary to construct a table for the special purpose of cutting up the large plates of glass into the sizes required. Any ordinary table about four feet long and three feet wide will be suitable for this purpose. Let the upper surface be planed perfectly level and smooth, and afterwards covered with green baize. Along two sides and one end a slip of hard wood is fixed upon the baize ; this slip is one inch wide and about one-sixteenth of an inch in thickness ; that is, slightly thinner than any glass plate that may be required to be cut. These slips are graduated into inches and fractional parts of an inch along the two sides, beginning at the angle in each case and proceeding to the left. A flat rule is next required, about three feet and a half in length, two inches wide, and half an inch thick ; it must be of 1

THE PHOTOGRAPHER’S GUIDE.

hard wood, and perfectly straight along the edges. Finally, a glazier’s diamond is the next requisite. The operator must go to a glazier, if he does not know how to use the diamond. He will learn more in five min¬ utes in this respect than we could impart to him by a lengthy description. The cut of a diamond on glass, when correctly made, is like the cut of a sharp knife when the edge of the blade is drawn along the surface of a piece of wood; it is a wedge-like incision totally distinct from a scratch. When the cut is made, the next thing is to separate the two parts ; this is effected by partly bending and partly pulling the two pieces assunder, beginning at one side and proceeding gradu¬ ally, though quickly onwards, to the other side. Nat¬ urally the incision made by the diamond must be complete to the edge, otherwise the fracture will not easily commence; and if the incision does not reach the opposite side, the fracture may deviate from a straight line at the part where the diamond cut is defective.

The operator has to find out in what position his diamond makes a cut, and not a scratch, and then he can ever afterwards fix it in this position.

The glass cutting operation is performed on and near the right end of the table; in consequence of this, the left end is comparatively free and may be set aside to receive the vice, etc., for polishing the plates with buckskin; whilst the drawer on the side will be a suitable receptacle for the diamond, files, buffers, buckskin, silk and linen cloths, and whatever else may be needed in this department. A certain portion of the drawer may be divided off into compartments for

THE PHOTOGRAPHER’S GUIDE.

the reception of the different sized plates used, as, for instance, for card-pictures, cabinet-pictures, etc.

The plates being now in readiness for the next step, the sharp edges are all abraded by means of a file kept for this purpose. "Whilst performing this work, the back of the operator is turned towards the table, in order that the fine particles of abraded glass may fall upon the floor and not upon the green baize on the table.

The next operation is to ascertain by actual trial that each abraded plate will fit in the shield of the plateholder ; for it is a very vexatious trouble to find, after the plate has been covered, with collodion and sensitized, that it is too large for the holder. After each trial, and when found correct in size, the plate is put away in its special receptacle nntil required.

Whether the plates have been used before, or not, we recommend that they be placed in a bath of nitric acid and water of the following strength :

Nitric acid - - - - -2 ounces.

Water - - - - - 2 pints.

They may remain in this bath for six or eight hours; by this steeping the old films of varnish and collodion are easily separated, whilst with fresh plates the sur¬ faces become thoroughly cleaned from adhering and extraneous substances. Instead of the preceding bath the following is also highly to be recommended:

CAREY LEA’S BATH.

Sulphuric acid - 2 ounces.

Bichromate of potash - - - 2 “

Water ------ 2 pints.

THE PHOTOGRAPHER’S GUIDE.

The result is about the same whichever bath is used.

The adhering films of varnish, collodion, and albu¬ men are likewise easily removed by immersing the plates in cold water and raising the latter to the boil¬ ing temperature. This is the least expensive method but not always the most convenient.

After the plates have thus received their first clean¬ ing, or the first preparation preparatory to washing, each one is taken and placed beneath the tap and sub¬ mitted to a thorough washing with a clean sponge or the fingers; the stream of water is finally allowed to play upon either side in succession, and then the plate is ready to receive a coating of albumen upon one side.

The best and easiest mode of preparing the albumen solution in stock is probably the one lately published by Mr. Ackland in his modified Fothergill process. The author says it may be preserved in good condition for years if kept closely corked.

“ Separate the yolks from the whites of any number of eggs, and to every eight ounces of albumen thus obtained add twenty drops of glacial acetic acid pre¬ viously diluted with one ounce of water; stir these to¬ gether with a glass rod until intimately mixed, which occupies about one minute; then, after resting one hour, strain through coarse muslin, and to the strained liquid add half a drachm of the strongest liquor ammonise; then preserve in carefully corked bottles.”

An ounce of this solution diluted with five ounces of water forms a liquid which is suitable for the sub¬ stratum on glass surfaces. Pour a sufficient quantity of this solution upon the bottom of a clean fiat porce-

THE PHOTOGRAPHER’S GUIDE.

lain dish so as just to cover two slips of glass placed along each end of the dish when lying fiat, and remove all bubbles with a small piece of paper.

Now take the plate of glass, which has just been thoroughly washed, and resting one end upon the slip of glass at the bottom of the dish, let the plate gradu¬ ally fall upon the surface of the albumen until the other end rests upon the second slip of glass. A silver hook will be found very convenient for lowering and raising the plate. Now raise the end, lift the plate up and examine its surface. The difficulty in this opera¬ tion consists in getting a clean surface free from all sort of particles. It frequently happens that such particles are met with on the first film or two. In this case, take the plate to the tap and wash off the albu¬ men thoroughly and begin the operation over again. As soon as the film is quite uniform and without flaw, the plate is allowed to drain for a moment and then put away on the rack to dry in a warm place where there is no current of air to produce dust.

The drying racks referred to are made of strips of corrugated iron; they are indispensable in the labora¬ tory, and can be had of any of our photographic deal¬ ers. Several of them will be needed where work is done systematically and with care.

The application of this substratum of albumen saves an immense quantity of labor in cleaning and polish¬ ing the glass surfaces by the old method with rotten stone and dilute alcohol; but this saving of labor is but a small part of the advantage gained. With the albumenous substratum, specks, stains, silver reductions on and under the collodion film will seldom or ever

THE PHOTOGRAPHER’S GUIDE.

occur; and we are not cognizant of a single instance where the collodion film slided off the plate when the albumen was beneath.

The film soon dries and is then indistinguishable from the glass surface as far as appearance goes; but the surface covered with this film can easily be distin¬ guished in case the plates have been misplaced. Glass itself is hygroscopic and retains the breath for some time, whilst, on the contrary, the breath is scarcely retained a moment on the side covered with the albumen.

In large galleries this operation fills up idle time; and in this way a large stock of prepared plates is al¬ ways on hand and ready at a moment’s notice for the next operation. But this brings us to the second chapter.

Chapter II

PREPARATION OF THE COLLODION ; AND THE MODE OF COATING THE ALBUMENIZED PLATES WITH COLLODION.

Collodion, for photographic purposes, consists of a solution of pyroxyline, or gun-cotton in a mixture of ether and alcohol, to which are added a certain quan¬ tity of soluble iodides and bromides, or of either alone according to the purposes for which it is intended. It is not advisable that the operator should manufacture his own gun-cotton ; for he can get it better and cheaper from the dealers, and especially from the chemists who devote their whole and sole attention to its preparation. Nevertheless, we will give our read¬ ers the formulas for the preparation of pyroxyline.

There are two distinct methods of making gun¬ cotton; the former is by means of a mixture of sul¬ phuric acid and nitrate of potash; and the latter of a mixture of sulphuric and of nitric acid. Cotton is im¬ mersed in either of these solutions and after remain¬ ing a short time it is taken out, washed, and dried. Apparently it has undergone no change ; but by weighing the quantity of cotton before and after the operation we shall find that, after this treatment, the weight has been increased. Furthermore, common cotton is insoluble in ether and alcohol; whilst gun¬ cotton or pyroxyline is quite soluble in this mixture. Before treatment, too, with the mixed acids cotton will burn, it is true, but it is not an explosive ingre-

THE PHOTOGRAPHER’S GUIDE.

dient, as it becomes after impregnation with one of the compounds, oxygen and nitrogen arising from the decomposition of nitric acid.

FIRST METHOD OF PREPARING PYROXYLINE.

Weigh out, in the first place, twelve ounces of pure nitrate of potash, pulverize it and dry it thoroughly before the fire; weigh also twenty-four ounces of sul¬ phuric acid such as is found in commerce; further¬ more, weigh out six drachms of clean and well-carded cotton. If the specific gravity of the sulpuric acid is 1.84, one ounce of water is required; if it is as low as 1.836, no water is required; and if it is higher than 1.84, two ounces of water may be measured out and kept in readiness. Separate the cotton into small tufts, and place them also in readiness.

Having next provided a large porcelain evaporating dish, fill it with hot water and let it remain until the vessel is quite warm on the outside. Pour the water out and wipe the dish. Now put in the nitrate of potash, then the water and stir the mixture about for a few seconds. The sulphuric acid is next added and intimately mixed. Finally, the cotton is immersed with rapidity, one tuft at a time, and pushed quite be¬ neath the surface of the mixture and so that every part may come in contact with the acid and salt. Work the cotton about, by means of two glass rods, for about four or five minutes, after the last tuft has been immersed; at the expiration of this time the cot¬ ton is taken out and thrown into a large pail of water, where it is well stirred about by means of the glass rods in order to get rid of the major part of the acid.

THE PHOTOGRAPHER’S GUIDE.

It is now immersed in a stream of running water, dis¬ integrated with the fingers, and washed for some time until all acidity has been removed. If a stream of water is not at hand, the washing operation is per¬ formed in several changes of fresh water; and finally, the pyroxyline, after a thorough wringing, is placed in a vessel of water, made alkaline with a few drops of ammonia. Separate the pyroxyline into small tufts in order that every part may come in contact with the alkali. After remaining in this water for several hours, test the liquid with blue litmus paper; if the latter is not changed in an hour’s time, it may be concluded that the acid has been entirely removed.

But now the pyroxyline has to be again washed in several changes of water in order to remove every trace of the alkali. This is a tedious operation, and can easily be dispensed with where there is a regular jet of water in the first instance to remove the acidity.

"Wring the pyroxyline, and then separate the matted tufts into thin layers, which may be dried in the sun or on a water bath maintained at a moderate tem¬ perature.

The weight of the pyroxyline will now be about eight drachms.

SECOND METHOD OF PREPARING PYROXYLINE, BY A MIXTURE OF NITRIC AND OF SULPHURIC ACID.

Make use, as before, of the large, glazed earthen¬ ware evaporating dish, which is first made warm by floating it on hot water; then pour into it the follow¬ ing acids:

1*

THE PHOTOGRAPHER’S GUIDE.

Nitric acid, sp. gr. 1.37 - 6 fluid ounces.

Sulphuric acid, sp. gr. 1.84 - 12 “

Stir the mixture and ascertain the temperature by means of a thermometer. When the thermometer has sunk as low as 150° Fahr., immerse six drachms of well-carded cotton, previously separated into small tufts as before. See that every tuft is thoroughly under the surface; now cover up the dish for ten minutes. At the expiration of this time the cotton is removed by means of the two glass rods; the excess of acid is well pressed out, and then the pyroxyline is plunged into a large bucket of water, and treated in the same way as before described in regard to pyroxy¬ line when prepared with nitrate of potassa, etc.

Ordinary nitric acid has the specific gravity 1.37, but it is well always to test the strength before com¬ mencing, because if it be stronger, the relative pro¬ portion of the two acids will be different.

Take the following as examples :

Nitric acid, sp. gr. 1.42 - 9 fluid ounces.

Sulphuric acid, sp. gr. 1.84 - 9 “

Cotton - 6 drachms.

Nitric acid, sp. gr. 1.40 - 8 fluid ounces.

Sulphuric acid, sp. gr. 1.84 - 16 “

Cotton - 6 drachms.

PREPARATION OF PLAIN COLLODION.

The pyroxyline when thoroughly washed and dried

THE PHOTOGRAPHER’S GUIDE.

may be preserved in pasteboard boxes for an indefi¬ nite time. In order to make collodion of it, it bas to be dissolved. It is well to keep a quantity of tbe dis¬ solved pyroxyline in stock, always ready for mixture with tbe sensitizing solution. Tbis stock solution is prepared as follows:

Sulphuric etber - - - 2 quarts.

Alcohol - - - - - 1 quart.

Pyroxyline - - - - 12 drachms.

Tbe pyroxyline soon dissolves in tbe above mixture, which is well shaken and then put aside in the cellar to settle and ripen. If the pyroxyline is pure and in good condition, there will not be much sediment. Whenever required for use, the proper quantity is decanted from the upper and clear portion at the top by means of a syphon or otherwise. This plain collo¬ dion has considerable consistence and may be diluted with alcohol contained in the iodizing solution.

PREPARATION OF BROMO-IODIZED COLLODION.

It is better to keep in stock a sufficient quantity of iodizing solution, by means of which a few ounces of bromo-iodized collodion may be readily prepared. Below will be found three different formulas for these stock solutions :

Formula No. 1.

Landscape Photography.

Iodide of ammonium Bromide of cadmium Alcohol

120 “

THE PHOTOGRAPHER^ GUIDE.

Formula No. 2.

Portrait Photography.

Iodide of cadmium - 182 grains.

Bromide of ammonium - 96 “

Formula No. 3.

Landscapes and Portraits.

Iodide of cadmium - Iodide of ammonium Bromide of ammonium - Alcohol -

48 “

Formula No. 4.

This formula contains no bromide.

Iodide of ammonium Iodide of cadmium - Chloride of magnesium Alcohol -

96 “

These stock solutions have to be kept in a cool dark place, together with the plain collodion; the cellar is a very suitable place. They remain in their present condition unchanged for an indefinite time.

PREPARATION OF IODIZED COLLODION.

Plain collodion - 3 ounces.

Iodizing solution - 1 ounce.

The first and second formulas contain five grains and a half of the mixed bromo-iodides in each ounce of solution; whilst the latter contains six grains to each ounce. The proportions of the salts in the two first

THE PHOTOGRAPHER’S GUIDE.

solutions are nearly those of their chemical equiva¬ lents.

The quantity of pyroxyline to each ounce of collo¬ dion, diluted as indicated by the alcohol in the bromo-iodizing solution, is six grains to the ounce. More or less can be added as circumstances require. A more consistent collodion may be used for very small plates; whilst a thinner collodion may be found necessary for large plates; that is, for all sizes above the four-fourth.

If the collodion is not quite clear and free from small floating particles, it will be found necessary to filter the solution before it can be used for coating the albumenized plates. Glass filters are manufactured for this special purpose, which allow the filtration to pro¬ ceed without exposing the collodion to the atmosphere. The glass tube with a small aperture at the closed end is intended to furnish a passage for air from the re¬ ceiver for a corresponding quantity of collodion that has filtered through the cotton, that surrounds it at the bottom, and that drops into the receiver. We prefer drawing the end of the glass tube into a capillary ter¬ mination of half or three-quarters of an inch long; this mode of construction succeeds in preventing the rising of the collodion up the tube instead of passing into the receiver, and of thus obstructing the filtration. These glass filters are very reliable and almost indis¬ pensable, if you aim at perfect productions.

In addition to the glass filters, collodion vials are kept for sale. They are constructed in such a manner as to prevent bubbles whilst the collodion is being poured out upon the plate. They are arranged, too, so as to allow all particles and specks of dry collodion to

THE PHOTOGRAPHER’S GUIDE.

accumulate around the neck and not to fall upon the collodion film. These cometless vials, as they are some¬ times called, are also very useful and reliable in the photographic laboratory.

Before the albumenized plate is coated with collo¬ dion, ascertain in the first place which side is covered with albumen, as previously indicated. Secondly, dust off all particles from this surface by means of a broad camel’s hair pencil, which must stand in a convenient position for this purpose, reared upon its handle. If the weather is damp, too, you had better move the plate a moment or two over a spirit lamp in order to drive off all moisture ; the latter may be seen as it volatilizes. Then let the plate cool. Hold the plate by the left front corner between the thumb and first finger of the left hand, whilst, if the plate be heavy, the second finger lends assistance in bearing the weight. The collodion vial is then taken in the right hand; now pour the collodion upon the plate at the right hand distant corner, and incline the plate gradu¬ ally whilst you are pouring, so that the collodion may flow to the other distant and left hand corner and then forwards toward the front. As soon as the plate is a little more than one-half covered, you stop pouring out any more collodion, and allow that, which has been already poured upon the plate, to proceed to the front, by inclining the plate, and so to accumulate near the right hand anterior corner, where the excess is allowed to flow back again into the vial. Whilst this excess is thus dropping off from the corner, oscillate the plate in the direction of the edge, as if it were poised on this corner; by doing so ridges on the collodion film in the

THE PHOTOGRAPHER’S GUIDE.

direction of the inclination, if the plate were quiescent, may be avoided. As soon as the collodion ceases to drop, or nearly ceases, raise the plate into the horizon¬ tal position, and oscillate it now in the direction of the flat surfaces; that is up and down gently; this motion removes all reticulated ridges that might have been formed by the preceding oscillatory motion; and the collodion surface becomes quite smooth.

As soon as the film is no longer soft, when touched with the finger, but yet not perfectly dry, the plate is immersed in the silver bath.

There is a considerable knack in coating a plate suc¬ cessfully with collodion, and this knack, like every me¬ chanical operation of this nature can scarcely be ob¬ tained otherwise than by practice. All that we can do is to point out the simplest and clearest method by which the art can be obtained.

To coat very large plates with collodion requires the aid of a pneumatic-holder, which contains at least two suction discs; one of three would still be more expedi¬ ent. This form of holder has just been patented; it is an excellent aid in such and similar operations, and can be attached in an instant to a plate by simply slap¬ ping the discs with a smart quick blow upon the oppo¬ site side of the plate to that which it is intended to coat with collodion. A holder with a single disc is sufficiently strong to hold any plate up to the four-fourth size. Another arrangement is found to be very efficacious in coating very large and heavy plates; plates that are much too heavy and too large to be supported by one hand. It is this : a strong walking cane, or, still better, the handle of a broom is fixed perpendicu-

THE PHOTOGRAPHER’S GUIDE.

larly in an aperture in the floor; the upper part is rounded off into a semi-spheroidal shape, and a piece of woolen cloth is tied over it. When about to coat the plate, the following is the proceeding: with both hands carry the plate and place it horizontally over the upper end of the broom-stick, so that the center of the plate rests on the woolen cloth; in such a position the plate is easily supported, and, being balanced, it can be os¬ cillated by means of the left hand in any direction whatever. The woolen cloth protects the surface from being scratched, and, at the same time, prevents the plate from sliding off its support. The collodion is poured upon the surface as before directed, and, as soon as the surface is completely covered, the collodion vial is laid aside, and the subsequent manipulations are performed with the two hands acting in combination.

If the collodion film is introduced into the silver bath before it has had time to congeal; that is, has be¬ come dry and compact in structure so as not to be dis¬ integrated by an impression from the finger, the film is liable to split up and break off in patches, or wholly, in the subsequent operations of developing, fixing, etc. On the other hand, if the film be too dry, it resists the permeation of the silver solution and does not become, in consequence, so easily and completely sensitized.

CHAPTEK III.

THE NITRATE OF SILVER BATH, ITS PREPARATION, TROUBLES, AND RENOVATION.

Nitrate of silver is a salt which is sometimes adul¬ terated; it is an advantage, therefore, to know how to detect this adulteration before the bath is prepared. Buy always the salt in the form of large flat crystalline tables ; if the crystals have more the appearance of common salt, it is a pretty certain indication that the salt is not pure.

To make the indication certain, dissolve five grains of the crystals in a drachm of water, and drop into the solution hydrochloric acid as long as a white curdy precipitate is produced; separate the chloride of silver, thus formed, by filtration through paper prepared for the purpose. Into the clear liquid filtrate add another drop of hydrochloric acid; if no curdiness is produced, it is a sign that all the silver has been removed from the solution, which, if the original salt was adulterated, will contain the salt or salts used in the adulteration, together with traces of hydrochloric acid.

Evaporate the filtrate to dryness; if nothing remains on the evaporating dish, it is an evidence that the silver salt was pure, unless nitrate of lead had been used in the adulteration; in this case the lead salt will be found on the filter. But if there is a whitish residue on the evaporating dish, this may be weighed and put down as so much impurity.

THE PHOTOGRAPHER’S GUIDE.

Mix a few particles of pulverized lime and some of the white residue together, and moisten the mixture with a drop of water. If nitrate of ammonia be pre¬ sent in this residue, it will be decomposed by the lime, and the fumes of ammonia will be set at liberty. The absence of all smell of ammonia shows distinctly the absence of an ammoniacal salt. In this case nitrate of soda may be suspected. This salt burns like salt¬ petre when thrown upon a piece of burning charcoal; and blotting paper impregnated with this salt in solu¬ tion and dried forms touch-paper, which, when kindled, burns with a deflagrating noise, but without flame, un¬ til it is totally consumed.

Supposing the impurity is nitrate of lead, it will be found now as chloride of lead mixed with chloride of silver on the filter. Filter through the mixed chlorides dilute nitric acid for some time. The chloride of lead will be dissolved, and pass through the filter, leaving the chloride of silver on the filter. Evaporate the so¬ lution to dryness; if there be no residue, there is no lead salt mixed with the silver salt.