The Boy's Playbook of Science Part 16

It was the great George Stephenson who asked the late Dean Buckland the posing question, "Can you tell me what is the power that is driving that train?" alluding to a train which happened to be pa.s.sing at the moment.

The learned dean answered, "I suppose it is one of your big engines."

"But what drives the engine?" "Oh, very likely a canny Newcastle driver." "What do you say to the light of the sun?" "How can that be?"

asked Buckland. "It is nothing else," said Stephenson. "It is light bottled up in the earth for tens of thousands of years; light, absorbed by plants and vegetables, being necessary for the condensation of carbon during the process of their growth, if it be not carbon in another form; and now, after being buried in the earth for long ages in fields of coal, that latent light is again brought forth and liberated, made to work--as in that locomotive--for great human purposes."

Such was the opinion of the most original and practical man that ever reasoned on philosophy; and could he have lived to realize the thorough adaptation and business use of light in the art of photography, he would have said, man is only imitating nature, and in producing photographs he must employ the same agent which in ages past a.s.sisted to produce the coal.

In another part of this elementary work we shall have to consider the nature of light; here, however, the chemical part only of the process of photography will be discussed.

Many years ago (in the year 1777) Jenny Lind's most learned countryman, Scheele, discovered that a substance termed chloride of silver, obtained by precipitating a solution of chloride of silver with one of salt, blackened much sooner in the violet rays than in any other part of the spectrum. He says, "Fix a gla.s.s prism at the window, and let the refracted sunbeams fall on the floor; in this coloured light put a paper strewed with luna cornua (horn silver or chloride of silver), and you will observe that this horn silver grows sooner black in the violet ray than in any of the other rays."

In 1779, Priestley directed especial attention to the action of light on plants; and the famous Saussure, following up these and other experiments, determined that the carbonic acid of plants was more generally decomposed into carbon and oxygen in the blue rays of the spectrum; these facts probably suggested the bold theory of Stephenson already alluded to. Pa.s.sing by the intermediate steps of photography, we come to the second year of the present century, and find in the Journal of the Royal Inst.i.tution a paper by Wedgwood, ent.i.tled "An Account of a Method of Copying Paintings upon Gla.s.s, and of making Profiles, by the Agency of Light upon Nitrate of Silver; with observations, by H. Davy."

Such a paper would lead the reader to suppose that very little remained to be effected, and that mere details would quickly establish the art; but in this case the experimentalists were doomed to [Page 139]

disappointment, as, after producing their photographs, they could not make them permanent; they had not yet discovered the means of _fixing_ the pictures. Nearly fourteen years elapsed, when the subject was again taken up by Niepce, of Chalons, with little success, so far as the fixing was concerned; and twenty-seven years had pa.s.sed away since the experiments of Wedgwood and Davy, when, in 1829, Niepce and Daguerre executed a deed of co-partners.h.i.+p for mutually investigating the matter.

These names would suggest a rapid progress; but, strange to relate, ten years again rolled away, the father Niepce had in the meantime died, and a new contract was made between the son and M. Daguerre, when, in January, 1839, the famous discovery was made known to the world, and in July of the same year the French Government granted a pension for life of six thousand francs to Daguerre, and four thousand to the son of Niepce, who had so worthily continued the experiments commenced by his father. The triumph of the industrious French experimentalists was not, however, to be unique; across the Channel another patient and laborious philosopher had completed on paper precisely the same kind of results as those obtained by Daguerre on silver plates. Mr. Fox Talbot, in England, had immortalized himself by a discovery which was at once called the Talbotype, and for which a patent was secured in 1841. Having thus hastily sketched a brief history of the art, we may now proceed to the details of the process.

_First Experiment._

A photogenic drawing, so called, but now termed _a positive copy_, is prepared by placing some carefully selected paper, which is free from spots or inequalities (good paper is now made by several English manufacturers, although some kinds of French paper, such as Cansan's, are in high repute), in a square white hard porcelain dish containing a solution of common salt in distilled water, 109 grains of salt to the pint. The paper is steeped in this solution for ten minutes, and then taken out and pressed in a _clean_ wooden press, or it should be dabbed dry on a _clean_ flat surface with a _clean_ piece of white calico, which may be kept specially for this duty and not used for anything else, and it is well that all would-be photographers should understand that neatness and cleanliness are perfectly indispensable in conducting these processes. If a design were required for the armorial bearings of the art of photography, it might certainly be most fanciful, but the motto must be _cleanliness_ and _neatness_, and in preparing paper it should not be unnecessarily handled, but lifted by the corners only. The object of dabbing the paper is to prevent the salt acc.u.mulating in large quant.i.ties in one part of the paper and the reverse in another, and to distribute the salt equally through the whole. The paper being now dried, is called salted paper, and is rendered sensitive when required by laying it down on a solution of ammonio-nitrate of silver, prepared by adding ammonia to a solution containing sixty grains of nitrate of silver to the ounce of distilled water, until the whole of the oxide of silver is re-dissolved, except [Page 140] a very small portion. A few drops of nitric acid are also recommended to be added, and after allowing the solution to stand, it may be poured off quite clear, and is ready for use either in the bath, or if economy must be rigidly adhered to, the salted paper may be laid flat on a board, and held in its place with four pins at the corners, and then just enough to wet the surface of the paper may be run along the side of a gla.s.s spreader, and the liquid gently drawn over the surface of the salted paper, which is allowed to dry on a flat surface for a few minutes, and afterwards hung up by one corner to a piece of tape stretched across the room, until quite dry, and then placed in a blotting-book fitting into a case which completely excludes the light. Copying-paper should be made at night, as the day is then free for all photographic operations requiring an abundance of light. It will not keep long, and should be used the next day.

[Ill.u.s.tration: Fig. 137. A. The gla.s.s spreader with cork handle. B. The silver solution clinging to rod and paper by capillary attraction. C C C C. Four pins holding down the paper on a board.--N.B. The spreader is made of gla.s.s rod three-eighths thick.]

A piece of lace, a skeleton leaf, a sharp engraving on thin paper, and above all things, a negative photograph on gla.s.s or paper, is easily copied by placing the prepared paper with the prepared side (carefully protected from the light) upwards on any flat surface, such as plate gla.s.s; upon this is arranged the bit of lace or the negative photograph with the face or picture downwards, another bit of plate gla.s.s is then placed over it, and weights arranged at the corners; after exposure to the sun's rays for thirty minutes, more or less (according to the dullness or bright aspect of the day), the picture is brought into a dark room and examined by the light of a candle or by the light from a window covered with yellow calico, and after placing a paper weight on one corner of the lace, or [Page 141] negative picture, or copying paper, it may be carefully lifted in one part, and if the copy is sufficiently dark, is ready for fixing, but if it is faint the lifted corner is carefully replaced, the upper gla.s.s is laid on, and the picture again exposed to the light. Should the position of the lace or negative be changed during the examination, re-exposure is useless, and would only produce a double and confused picture, as it would be impossible to lay the lace or the negative exactly in the same place again on the copying paper.

The manipulations just described are much facilitated by using a copying-frame or press, which consists of a square wooden frame with a thick plate-gla.s.s window; upon this are placed the negative picture and the copying paper, and the two are brought in close contact by means of a board at the back pressed by a hand-screw. (Fig. 138.) After the photogenic drawing or positive copy is taken, it is fixed by being placed in a solution of hyposulphite of soda, consisting of one fluid ounce of saturated solution to eight of water. The saturated solution of hyposulphite of soda is conveniently kept in a large bottle for use, and in order to improve the colour a very little chloride of gold is added to the fixing solution, the picture must now be thoroughly washed, dried, and pressed.

[Ill.u.s.tration: Fig. 138. The back of the copying-frame, showing the hand-screw and pressure-board. The plate gla.s.s inside is set in the base of the frame, and is of course the part exposed to the light.]

_Second Experiment._

Another mode of preparing the copying paper, called alb.u.men paper, is to take the whites of four eggs, and four ounces of distilled water containing one hundred and sixty grains of chloride of ammonium; these are beaten up with a fork or a bundle of feathers, and as the froth is produced it is skimmed off by a silver spoon into another basin, or a beaker gla.s.s, and being allowed to settle for twelve hours it is strained through fine muslin, and is ready for use. The best paper is floated on the surface of this liquid for three minutes, taken out, and dried at once on a hot plate.

In floating paper one corner is first laid down, and care taken not to enclose any air bubbles, which would prevent the fluid wetting the paper, whilst the remainder of the paper is slowly laid upon the surface of the fluid.

The alb.u.men paper is excited by laying it for five minutes on a solution of nitrate of silver, seventy-two grains to the ounce of water, [Page 142] and when dry it will keep for three days. This copying paper is used in the same manner as the last, and fresh eggs only must be used in its preparation, because stale ones soon cause the copy to change and blacken all over from the liberation of sulphur, which unites with the silver. The colour of the copy is sometimes improved by a solution of hot potash, and by dipping the well-washed picture, after the use of the hyposulphite of soda, in a very dilute solution of hydrosulphuret of ammonia.

_Third Experiment._

In the Daguerreotype process, a silver plate, after being thoroughly cleaned and polished, is exposed to the vapour of iodine, and is thus rendered so sensitive that it may be at once exposed in the camera. In the Talbotype process, the same principle is apparent, and paper is prepared by first covering its surface with iodide of silver, which is afterwards rendered sensitive to the action of light by means of an excess of nitrate of silver, as follows:--

One side of a sheet of selected Cansan's paper is first covered (by means of a spreader) with a solution of nitrate of silver (thirty grains to the ounce of water), hung up in a dark room and dried; it is then immersed in a solution of iodide of pota.s.sium of five hundred grains to a pint of distilled water, for five or ten minutes, and immediately changes to a yellow colour in consequence of the precipitation of the yellow iodide of silver; it is then well washed with plenty of water, and being dried, may be kept for any length of time, and is called "iodized paper." Light has no action whatever upon it. To render the paper sensitive, three solutions are prepared in separate bottles, and marked 1, 2, 3.

No. 1, contains a solution of nitrate of silver, fifty grains to the ounce of water.

No. 2, glacial acetic acid.

No. 3, a saturated solution of gallic acid.

With respect to No. 3, Mr. William Crookes has shown, that when a saturated solution of gallic acid is required in large quant.i.ties, that it is better to dissolve at once two ounces of gallic acid in six ounces of alcohol (60 over proof); to hasten solution, the flask may be conveniently heated by immersion in hot water; when cold it should be filtered, mixed with half a drachm of glacial acetic acid, and preserved in a stoppered bottle for use; so prepared it will keep unaltered for a considerable length of time. The gallic acid is not precipitated from this solution by the addition of water; consequently, if in any case desirable, the development of a picture may be effected with a much stronger bath than the one usually employed. To obtain a solution of about the same strength as a saturated aqueous solution, such as No. 3, half a drachm of the alcoholic solution is mixed with two ounces of water; but for my particular purpose, says Mr. Crookes, referring to the wax-paper process, "I prefer a weaker bath, which is prepared by mixing half a drachm with ten ounces of water." In either case it [Page 143]

will be found necessary to add solution of nitrate of silver in small quant.i.ties, as the developing picture seems to require it.

Returning again to the solutions marked 1, 2, 3, the numbers will a.s.sist the memory in mixing the proportions of each. If the paper is required to be used _at once_, a drachm of each may be mixed together and spread over the iodized paper (of course, in a dark room), which is then transferred to a clean blotting-book of white bibulous paper, and being placed in the paper-holder may be taken to the camera and exposed at once. If the paper is not required to be used immediately, the solutions are mixed in the proportions of the numbers--viz., one of No. 1, two of No. 2, three of No. 3; and in making the mixture, it is advisable to keep a measure specially for No. 3, the gallic acid, or else the measure, if used for the three solutions, will have to be washed out every time, which is very troublesome, particularly where water is not plentiful.

If the excited paper is required to be kept some hours before use, No. 3 must be added in still larger proportion, as much as ten or even twenty measures of No. 3 to two of No. 2, and one of No. 1, being used, and even this large dilution is frequently insufficient to prevent the paper spoiling in hot weather; therefore if the temperature is high, too much reliance must not be placed on this paper, as it is peculiarly disappointing, after walking some miles to romantic and beautiful scenery, to find, when developing the pictures in the evening, that the paper used was all spoilt before exposure; and it will be seen presently that when the excited paper is to be carried about for use, it is better to adopt the wax-paper process.

After the excited iodized paper is exposed in the camera--and the time of exposure cannot be taught, as that speciality is only acquired by experience, and may vary from five to thirty minutes, or even more--the invisible picture is developed and rendered visible, not by exposure to the vapour of mercury, as in Daguerre's process with silver plates, but by a mixture of one of No. 1 with four of No. 3. The development is carefully watched by looking through the negative placed before a lighted candle, and the time of development may vary from ten to thirty minutes, and all the time the picture must be kept wet with the solution, so that it is better perhaps to make a bath of the solution and lay the picture on its surface than to pour the liquid over the picture. After the development is matured, the picture is now washed in clean water, and fixed temporarily, if required, by immersion in a bath containing 200 grains of bromide of pota.s.sium in one pint of water, or permanently by the hyposulphite of soda, made by mixing one part of a saturated solution with five or ten of water, or one ounce of the salt to six or twelve of water; but, as before mentioned, it is better to keep a Winchester quart full of a saturated solution of hyposulphite of soda, and then it is always ready for use instead of employing the weights and scales, and continually weighing out portions of the salt.

The picture after fixing is thoroughly washed with water, and being [Page 144] dried is now placed between the folds of a wax book--_i.e._, some leaves of blotting-paper are kept saturated with white wax, and when a picture is placed between them, and a hot iron pa.s.sed over the outside sheet, the wax enters the pores of the paper, and after removing any excess of wax by pa.s.sing the picture through a book of bibulous paper, over which the hot flat iron is pa.s.sed, the negative picture at last is ready for use, and any number of positive copies may be taken from it, as already described in the first experiment, page 139.

This mode of manipulation is called the Talbotype, and before dismissing the subject another process of iodizing the paper may be explained.

To a solution of nitrate of silver of twenty, thirty, or fifty grains to the ounce of water, a sufficient number of the crystals of iodide of pota.s.sium is added, first to produce the yellow iodide of silver, and then to dissolve it, so that the yellow precipitate appears with a small quant.i.ty, and disappears with an excess of the iodide. If this solution is spread over sheets of paper, and these latter then placed in a bath of water, the iodide of silver is precipitated on the surface, and after plenty of was.h.i.+ng to remove the excess of iodide of pota.s.sium, the paper may be dried, and will keep for any length of time without change. This paper may be excited, exposed, developed, fixed, and waxed, as already explained.

_Fourth Experiment. The Wax-paper Process._

This mode of taking negative photographs begins where the talbotype ends--viz., by _first_ waxing the paper perfectly and evenly, as already explained, Cansan's negative paper being preferred. The wax paper is now well soaked in a bath, made by dissolving one hundred grains of iodide of pota.s.sium, six grains of cyanide of pota.s.sium, four grains of fluoride of pota.s.sium, ten grains of bromide of pota.s.sium, ten grains of chloride of sodium, in one pint of fresh whey, with the addition of a little alcohol and a few grains of iodine. When soaked in this solution for about one hour, the paper is taken out and hung up to dry.

N.B. With respect to iodizing the wax paper, it is almost better to obtain it ready prepared, and then every sheet may be relied on. Mr.

Melhuish, of Blackheath and Holborn, supplies it in any quant.i.ty, and his paper never fails; the operator has then only to perform the sensitizing and developing processes. To render the iodized paper sensitive it is immersed for about six minutes in a bath containing a solution of nitrate of silver (thirty-five grains to the ounce of water, with forty drops of glacial acetic acid); the paper is now removed, and washed in two trays of common clear rain-water or distilled water, and is then dried off between folds of blotting-paper.

This process may be performed on the previous evening by the light of a candle, or by day in a room lit by one window covered with four thicknesses of yellow calico, and after the paper is dry it will keep for three [Page 145] weeks or a month, and may be exposed in a camera with a three-inch lens of eighteen-inch focus, with the inch diaphragm, on a bright day from five to fifteen minutes; in bad weather the exposure must be longer. The picture may be carried home and rendered visible or developed by immersion in a bath containing a saturated solution of gallic acid, and as the developing continues, a few drops of the sensitizing solution of nitrate of silver and glacial acetic acid may be added. Finally, the picture is fixed by immersion for a quarter of an hour in a solution of hyposulphite of soda (four ounces of the crystal to one pint of water, or one part of the saturated solution to eight of water), and being well washed, is then dried, hung before the fire to melt the wax, and is now ready to print from.

_Fifth Experiment. Alb.u.men on Gla.s.s Process._

Alb.u.men is the scientific name for the white of egg, of which four ounces by measure are mixed with one ounce and a half of distilled water, and after being whisked to a froth, are removed by a spoon into another basin or a beaker gla.s.s, and allowed to stand for several hours and then filtered. Mr. Crookes has recommended a very ingenious, simple, and useful filter. (Fig. 139.) He says: "This simple and inexpensive piece of apparatus, which any instrument maker or gla.s.s-blower can supply at a few hours' notice, will be found invaluable in almost every photographic process on gla.s.s. The sponge has this great advantage over all other kinds of filters, that thick gelatinous liquids--_e.g._, honey, alb.u.men, gelatine, meta-gelatine, or the various preservative syrups--flow through it with the utmost readiness; whilst at the same time dust, air bubbles, or froth, and dried particles floating in the liquid, are effectually kept back, and if fitted with stoppers, collodion might be filtered in it; or if the ends were fitted together with a bit of flexible pipe, the stoppers might be dispensed with altogether.

[Ill.u.s.tration: Fig. 139. A B. Gla.s.s tube, bent as in picture. C. Piece of damp sponge squeezed into the head of the tube. Any liquid poured in at B will flow through the sponge until it has attained the same level in A.]

Having poured the alb.u.men on a perfectly clean gla.s.s plate, taking care to have sufficient to run freely over the surface of the gla.s.s, the excess is then gently drained off and the plate turned so as to have the coated side downwards; it is then fixed in a sling made by taking a stout bit of string about three feet long, which is doubled and knotted at the fold, leaving the two ends free; two small triangles or stirrups of silver wire looped at one corner are now tied on to the ends of the string, and these form a support for the opposite edges of the gla.s.s plate to rest on; the two strings are knotted together at a [Page 146]

convenient distance from the stirrups to prevent the gla.s.s slipping out, and the plate is now rotated rapidly over a heated metallic surface, such as an iron box containing some burning charcoal or the _warming pan_, care being taken to avoid dust as much as possible, and to use only the whites of new-laid eggs. (Fig. 140.) The gla.s.s plate, covered with dry alb.u.men, is now iodized to a straw colour by exposure over a box containing iodine, as in the Daguerreotype process, and is sensitized by immersion for three or four minutes in a bath containing a solution of nitrate of silver (twenty-five grains to an ounce of water); the plate is afterwards washed in distilled water and left to dry spontaneously, of course in a darkened room. The plates may then be placed ready for use in a very ingenious tin box devised by Mr. Crookes, which keeps them perfectly light-tight even in the sun, and at the same time is less bulky than the ordinary wooden ones. It is made of tin plate, the cover sliding tight over the top, and more than half way down the sides; light is further excluded by means of an outer jacket of tin, which is soldered to the box a little below the centre. The cover thus slides between the case and the jacket, and renders injury to the plates by the entrance of light an impossibility. (Fig. 141.)

[Ill.u.s.tration: Fig. 140. A. Loop for finger. B. The knot which prevents the stirrups of silver wire, C C, slipping off the corners of the gla.s.s plate. D D. The opposite corners of the gla.s.s plate on which the stirrups are placed.]

[Ill.u.s.tration: Fig. 141. A A. Tin box, with part.i.tions to hold gla.s.s plates, B B. The outer jacket, between which and the box, A, the lid or cover, C, slides.]

The sensitive alb.u.menized gla.s.s plate is exposed in the camera from fifteen to thirty minutes, and developed (much in the same way as the paper pictures) with one ounce of a saturated [Page 147] solution of gallic acid containing ten or fifteen drops of the sensitizing solution.

The plate is usually placed on a levelling stand, and the solution poured on the gla.s.s plate; the development is slow, and may be quickened sometimes by the application of heat.

The picture is fixed by immersion for a short time in a bath containing one part of a saturated solution of hyposulphite of soda in eight of water. The pictures produced by this process are exquisitely defined, provided always the camera is well focussed, and to a.s.sist this operation a magnifying gla.s.s may be employed. After removal from the hyposulphite of soda the plate is well washed with water, and being allowed to dry spontaneously, is now ready to print from.

_Sixth Experiment. The Collodion on Gla.s.s Process._

The gla.s.s plates for this, as well as the alb.u.men on gla.s.s process, should be cleaned by rubbing them over first with a mixture of Tripoli powder and ammonia, which is washed off under a tap, and the gla.s.s being drained is rubbed dry and polished with a clean calico duster kept exclusively for this purpose.