The Boy's Playbook of Science / Including the Various Manipulations and Arrangements of Chemical and Philosophical Apparatus Required for the Successful Performance of Scientific Experiments in Illustration of the Elementary Branches of Chemistry and Natural Philosophy

by John Henry Pepper

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 assisted 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 glass 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. Passing by the intermediate steps of photography, we come to the second year of the present century, and find in the Journal of the Royal Institution a paper by Wedgwood, entitled "An Account of a Method of Copying Paintings upon Glass, 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 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 Niépcè, of Chalons, with little success, so far as the fixing was concerned; and twenty-seven years had passed away since the experiments of Wedgwood and Davy, when, in 1829, Niépcè and Daguerre executed a deed of co-partnership for mutually investigating the matter. These names would suggest a rapid progress; but, strange to relate, ten years again rolled away, the father Niépcè 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 Niépcè, 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 accumulating in large quantities 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 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 glass 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.

Fig. 137.

a. The glass 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 glass 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 glass 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 glass; upon this is arranged the bit of lace or the negative photograph with the face or picture downwards, another bit of plate glass 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 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 glass 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.