The 1860s were a pivotal period in the history of chemistry, bridging the age of isolated discovery and the establishment of a coherent scientific framework. Chemists possessed a significant collection of fundamental substances but lacked a universal language or organizational structure to make sense of them. Intense experimentation led to the rapid identification of new elements, creating disorder in how the scientific community communicated findings. Defining what an element was, its weight, and its relationship to others became an urgent international problem that needed solving before any grand classification system could emerge.
The Number of Known Elements
By the beginning of the 1860s, the scientific community recognized approximately 60 to 63 distinct chemical elements. The exact count fluctuated due to ongoing disputes and the difficulty of isolating rare substances. The discovery rate had accelerated in the mid-19th century, particularly with the advent of new analytical tools like the spectroscope.
Newly developed spectroscopic analysis quickly led to the identification of several elements, contributing to the ambiguity in the exact number. Caesium and Rubidium were discovered by Robert Bunsen and Gustav Kirchhoff in 1860 and 1861, respectively, using this new method. Thallium was discovered in 1861, and Indium followed a few years later in 1863. Although the existence of these elements was confirmed, their properties and atomic weights were not yet universally agreed upon, making a firm count difficult. When Dmitri Mendeleev began his work in the late 1860s, he worked with a set of 63 recognized elements, which served as the foundation for his organizational efforts.
Standardization and the 1860 Karlsruhe Congress
The primary obstacle preventing a consistent count and classification was the lack of agreement on fundamental chemical concepts. Before 1860, chemists used inconsistent systems for determining atomic weights, the relative masses of atoms. This meant that the same chemical compound, such as water, could be assigned different formulas and calculated weights, leading to profound confusion in published research.
To resolve this disorder, the First International Congress of Chemists was convened in Karlsruhe, Germany, in September 1860. This landmark meeting brought together over 140 leading chemists to discuss the standardization of nomenclature, notation, and atomic weights. Although the conference did not end with an immediate, formal vote, a decisive turning point occurred with the distribution of a paper by the Italian chemist Stanislao Cannizzaro.
Cannizzaro’s work revived and applied the hypothesis of Amedeo Avogadro, proposing that equal volumes of gases contain an equal number of molecules under the same conditions. By applying this principle, Cannizzaro provided a systematic method for distinguishing between an atom and a molecule, and for assigning correct, relative atomic weights. This new, standardized system, which set the atomic weight of hydrogen at 1, helped resolve decades of confusion and provided the foundation for creating a unified system of element classification.
Organizing the Known Elements
With the number of recognized elements hovering above sixty and a reliable system of atomic weights finally available, the scientific focus shifted to finding a rational way to organize them. The volume of elements presented a challenge, prompting chemists to seek patterns to manage this large dataset. Several scientists attempted to arrange the elements based on their chemical and physical properties, using the standardized atomic weights as the primary ordering principle.
One significant attempt came from the English chemist John Newlands, who proposed his Law of Octaves in 1865. Newlands observed that when elements were arranged in order of increasing atomic weight, every eighth element exhibited similar properties, much like the notes in a musical scale. His system, which classified 56 elements, was initially met with skepticism but demonstrated an early recognition of periodicity.
Around the same time, the German chemist Lothar Meyer was also working on a comprehensive classification system. Meyer’s effort in 1868 involved a table of 56 elements based on the periodic repetition of physical properties, such as atomic volume, when plotted against atomic weight. The independent development of similar arrangements underscored the scientific community’s need for a predictive, universally accepted system to manage the growing list of known elements.