PhotoHistory

From: PHOTOGRAPHY OF TO-DAY

By H. CHAPMAN JONES, 1913

The Development of Photography (part 2 of 3)

Silver nitrate

Silver nitrate at this time had been known for hundreds of years both in the solid form and in solution, and it was used in surgery. It was a familiar substance among those interested in such matters, and it is not surprising therefore to find that it was recommended for use as a secret ink about the time that we are now considering. By writing with its solution on paper in a subdued light, the inscription was invisible, but on exposure to light it would gradually grow dark. Not a very practical or safe secret ink, but the proposal to use it for this purpose shows that the action of light on silver compounds was becoming more widely known and that there was a tendency to apply it to useful purposes.

Charles William Scheele, [Carl Wilhelm Scheele] a Swede, who had been successful in isolating chlorine from hydrochloric acid three years before, in 1777 investigated the change that light effects in chloride of silver. Whether or not the alchemists knew that silver chloride became darkened by exposure to sunlight, it was before this a well ascertained and recorded fact, and Scheele, who was an indefatigable investigator, was interested in the action of light in general and especially in its relation to the action of heat upon substances. He prepared a quantity of silver chloride, dried it, and exposed it to sunshine for two weeks, stirred it up and exposed it again, and so continued until he obtained a practically black powder. This he treated with ammonia, which dissolves silver chloride, and obtained a black insoluble residue which he identified as silver. Hence he concluded that the action of light consisted in the separation of metallic silver from the chloride of silver. In another experiment he exposed to the light some chloride of silver in water, and he found that the water contained the chlorine that the light had separated from the silver salt. The inferences drawn from these observations we know now to need a little modification, but in the main they are correct. Scheele went even further than this, for he coated some paper with chloride of silver and found that violet light acted upon it to darken it more quickly than any other rays of the sun’s spectrum.

Experiments of the kind just described were made by other investigators, some of whom carried their researches further than their predecessors, but we are not particularly concerned with them. Our present aim is to get an idea as to how matters stood at the end of the eighteenth century, that we may better understand the attitude of those whose work is now to be referred to. The sensitiveness of various silver compounds to light was then well known, silver salts had been spread upon paper for the purposes of investigation, and the character of the change produced by light had been discovered by an examination of the products in the case of silver chloride as well as the nature of the active light.

We may not think much of a suggestion made by Lord Brougham in 1795 when he was a youth of seventeen, to rub silver nitrate on ivory and on the surface to receive and so render “permanent” the picture produced by the camera. This suggestion was deleted from a communication that Brougham sent to the Royal Society, and he does not appear to have considered that it was worth taking further trouble about.

Thomas Wedgwood, the fourth son of Josiah Wedgwood the renowned potter, was born in 1771 and suffered all his short life (he died at thirty-four years of age) from ill-health which gradually unfitted him for any work except traveling in the endeavor to mitigate his sufferings. He had a distinct taste for scientific pursuits, and was interested especially in the effects produced by light and heat. He worked with a solar microscope, that is an apparatus that utilizes the direct rays of the sun to give on a screen an enlarged image of suitable small objects, and he became anxious to make “permanent” the images so produced. The word permanent here does not mean that the picture should of necessity last for long, but only that the picture should be so “fixed” on the screen that it should remain after the apparatus producing it was removed. For this purpose he used silver salts, especially the nitrate, and probably worked on and off as circumstances permitted for some years in his endeavors to get satisfactory results.

Sir Humphry Davy began to study chemistry in 1798, in the following year he began an important series of experiments on the inhalation of nitrous oxide, and in 1801 he left an appointment that he held at Bristol to come to the newly established Royal Institution in London. When Davy came to London he was a young man twenty-three years of age and Wedgwood was twenty-nine. Wedgwood showed Davy his results either in this or the following year, perhaps seeking the advice of the young chemist with regard to some means for preventing the light from obliterating his photographs by blackening all over the paper on which he had produced them. All that is really known is that in 1802 there was published in the Journal of the Royal Institution a communication with the title “On an account of a method of copying paintings upon glass, and of making profiles, by the agency of light upon nitrate of silver. Invented by T. Wedgwood, Esqr. With observations by H. Davy.” Paper or leather was moistened with a solution of nitrate of silver and the image allowed to fall upon it. The color of the impression produced could not be washed out with soap and water, but all attempts by repeated washing or varnishing to render the uncolored portion of the paper indifferent to light, to “fix” the print as we should now say, were unsuccessful. The hyposulphites, the ordinary fixing salt of to-day, had been discovered three years before, but it was only a laboratory curiosity and perhaps it had not come under Davy’s observation. Leaves, insects’ wings, and ordinary printing press prints could be copied slowly by putting the sensitive paper in contact with the original and exposing to light, but a camera image was too faint to produce a result. Davy says that to secure the camera image was the first object of Mr. Wedgwood, but that in this all his numerous experiments were unsuccessful. Davy tried silver chloride and found it more sensitive than the nitrate. He refers to some possible future experiments with regard to “destroying” the sensitive compound not acted on by light (fixing), and adds that this is all that is needed “to render the process as useful as it is elegant.” These further experiments do not seem to have been made. Three years after this Wedgwood died, and Davy, who was busy with the duties of his position and his chemical investigations, probably gave no further thought to the matter. In 1829 Davy died.