The history of photography is inseparably linked with research in chemistry, especially the study of light sensitivity of various substances. Johann Heinrich Schulze (1687-1744) became perhaps the first scientist who consistently and systematically studied the light sensitivity of silver salts.
Silver nitrate was known to medieval alchemists. Centuries before the invention of photography, this compound, called lapis or lapis infernalis (which translates as “infernal stone”), attracted close attention from scientists. They were interested in its antiseptic properties. Silver nitrate in low concentrations can suppress the activity of microorganisms and provide anti-inflammatory effects, but in high concentrations, it can cauterize skin.
One of the first to seriously study the properties of silver salts was German scientist Johann Heinrich Schulze. He was a true polymath of his time: a doctor, chemist, philosopher, and also a professor of anatomy at the University of Halle.
Schulze conducted his main experiments in 1717 when he first noticed that a mixture of silver nitrate and chalk darkens when exposed to light. This became his starting point for further research. A few years later, in 1727, Schulze published the results of his experiments, where he described in detail the reaction of silver nitrate to light. Previously, it was believed that darkening occurred due to heat. But Schulze showed that the cause was sunlight.
One of Schulze’s key discoveries was the study of silver chloride, which formed when silver nitrate combined with hydrochloric acid. Silver chloride precipitated as a white residue that could be washed and filtered. Schulze discovered that the new substance also changed color when exposed to light.
Schulze used this effect in demonstrations — by placing stencils with text on bottles with silver nitrate solutions, he achieved that under the influence of light, words would “appear” on the solution. Although the images were temporary and disappeared with further exposure to light, these experiments became an important stage in the development of photochemistry and later served as the basis for the work of other scientists, such as Thomas Wedgwood and Humphry Davy.
Schulze made a real breakthrough, proving that light is not just what we see. It can change the properties of substances. However, he was unable to preserve his images for long. But his discoveries became the foundation for future inventors of photography. This discovery became the starting point for such well-known photographic processes as daguerreotype and calotype, which appeared in the 19th century.