Mathew Carey Lea: Chemist, Photographic Scientist

Mathew Carey Lea. Courtesy of M. Susan Barger

By David Whitcomb

When Mathew Carey Lea’s groundbreaking paper on colloidal silver appeared in the American Journal of Science in 1889, it bore an unusual accompaniment. The journal’s editors—impressed with the silver-coated glass and paper samples that Lea had sent them—inserted a comment at the end of the article congratulating him on his “very important results”: “The coated strips, including the gold-colored mirror made with the ‘gold-silver,’ answer fully to his description. The mirror is remarkable for its perfection and brilliancy.” What exactly had they seen, and what made his findings so “remarkable”?

Carey Lea (as he was normally addressed) had discovered what he called three allotropic forms of silver: “soluble silver,” “insoluble silver,” and “gold-yellow or copper-colored silver.” He recognized that these allotropic forms, which formed colloids containing particles too small to be seen in any microscope of the day, suggested numerous applications. He found, on the one hand, that he could create a metallic silver film equal to any mirror prepared by conventional techniques simply by evaporating a dispersion of either soluble or insoluble silver on a smooth surface. If he used a different “form” of silver, on the other hand, he could produce a material visually indistinguishable from gold—indispensable evidence for those still clinging to a belief in the chemical transmutation of elements. Who would have thought that it would be possible to repeatedly “dissolve” and precipitate metallic silver without any apparent adverse effects? It was this work, and his detailed study of photography, that established his reputation worldwide and caught the attention of George Eastman, the founder of the Eastman Kodak Company.

Finding a Path to Science

Carey Lea (1823–1897), the son of a leading Philadelphia family, almost became a lawyer instead of a scientist. He studied law with the prestigious W. M. Meredith firm and was admitted to the bar at age 24, but continued health problems forced him to change direction. After seven months touring Europe with his father, he resigned himself to the life of an invalid and returned to Philadelphia.

Back at home in suburban Chestnut Hill, Lea soon returned to an earlier interest: chemistry. Neither Lea nor his brother, Henry, who would gain prominence in his own right as a medieval historian, attended school or university; instead, they were taught by a series of tutors hired by their father, the prolific naturalist and publisher Isaac Lea. When Isaac felt his sons’ basic education in languages and the sciences was complete, he enrolled both Henry and Carey in James Curtis Booth’s recently opened private instructional laboratory. The choice of the Booth laboratory was fortuitous in that it provided a unique opportunity for personal instruction by an acclaimed chemist in an applied setting.

Booth was the first American chemist to expand his training by studying in renowned European laboratories. His unusual teaching laboratory also functioned as a chemical consulting business with a focus on analytical chemistry. Booth was a particularly careful educator and writer; among his most important contributions was the extraordinarily important, 1,000-page Encyclopedia of Chemistry, Practical and Theoretical (Philadelphia, 1850), which became the chemistry reference book of choice for over 50 years. His analytical capabilities and precision were held in such high regard that President Zachary Taylor appointed him melter and refiner of the U.S. Mint in Philadelphia (1849). He would eventually also serve as president of the fledgling American Chemical Society (1883–1885). The few years that Lea studied with Booth provided the practical analytical chemistry training that would be the key to his future success. Indeed, his first scientific publication, an analysis of Pennsylvania coals undertaken at his father’s request, was the result of work conducted in Booth’s laboratory.

Upon returning to Philadelphia from Europe Lea built a private laboratory at his home where he could explore his diverse interests in organic chemistry, biological systems, and photography. Prior to his near total immersion into this last area, he developed an interest in the organic chemistry of picric acid, used primarily at that time as an analytical reagent. Around 1849 or 1850 he lost an eye from a laboratory accident, perhaps from a picric acid explosion—the compound is a close relative of TNT. He published a series of papers (1858–1861) that expanded on the explosive sensitivity of this compound and its metal salt derivatives, which detonate or “deflagrate with great heat and light intensity” upon heating. Although Lea recognized the potential military uses of such an explosive and recommended it to the U.S. government early in the Civil War (most likely via the nearby Frankford Arsenal), the government apparently declined the suggestion. Picric acid was later patented as an explosive for mining applications and was eventually tested as a military explosive before TNT came into widespread use.

Photographic Scientist and Pioneer

Although Lea published articles in many areas of analytical and physical chemistry, his investigations into photography and photographic chemistry were by far his most important contributions to science. Born into a wealthy family, he had begun experimenting with photography as early as 1840, not long after Louis Jacques Mandé Daguerre and William Henry Fox Talbot announced the discoveries that introduced the first practical ways to capture images permanently. At age 17, at his father’s request, he prepared photographs of nearly 40 specimens of plants and shells for exhibition at the American Philosophical Society. These beautiful photographs, based on a potassium dichromate process, required considerable artistic talent as well as scientific skills.

By the 1860s, Lea was directing much of his attention to the science of photography. He compiled his experiences and research in photography into a practical text entitled A Manual of Photography: Intended as a Text Book for Beginners and a Book of Reference for Advanced Photographers (Philadelphia, 1868). A completely revised edition, incorporating over 100 new pages and twice the number of engravings, was printed in 1871 and is still available in many libraries today.

The Manual was significant for several reasons. First, it included detailed chemical recipes for making photographic emulsions and developing them. The volume offered advice on both exposure times and optimizing camera conditions for specific types of pictures. Lea illustrated a wide variety of imaging techniques for both landscapes and portraits, and he advised readers on how best to use the newly introduced magnesium flash lighting. A more unique contribution focused on techniques for color photography, then known as heliography, which relied on the conversion of the lightsensitive component of the film (silver chloride or silver bromide) to generate different shades of color from metallic silver. The main problem of how to make the colored image permanent would not be solved for another 50 years. In addition, he provided highlights of such specialized techniques as stereophotography and microphotography. Lea even included an entire section on photography and health—an important if not radical idea for the time—showing concern for the photographer’s health and safety. He warned the photographer that several of the reagents, such as potassium cyanide, decomposing nitrocellulose, and nitric acid, were strong poisons, and he offered advice on how to use them safely.